    def _forward_sam_heads(
        self,
        backbone_features,
        point_inputs=None,
        mask_inputs=None,
        high_res_features=None,
        multimask_output=False,
        ## Extension: LLM prompt
        language_embd=None,
    ):
        """
        Forward SAM prompt encoders and mask heads.

        Inputs:
        - backbone_features: image features of [B, C, H, W] shape
        - point_inputs: a dictionary with "point_coords" and "point_labels", where
          1) "point_coords" has [B, P, 2] shape and float32 dtype and contains the
             absolute pixel-unit coordinate in (x, y) format of the P input points
          2) "point_labels" has shape [B, P] and int32 dtype, where 1 means
             positive clicks, 0 means negative clicks, and -1 means padding
        - mask_inputs: a mask of [B, 1, H*16, W*16] shape, float or bool, with the
          same spatial size as the image.
        - high_res_features: either 1) None or 2) or a list of length 2 containing
          two feature maps of [B, C, 4*H, 4*W] and [B, C, 2*H, 2*W] shapes respectively,
          which will be used as high-resolution feature maps for SAM decoder.
        - multimask_output: if it's True, we output 3 candidate masks and their 3
          corresponding IoU estimates, and if it's False, we output only 1 mask and
          its corresponding IoU estimate.

        Outputs:
        - low_res_multimasks: [B, M, H*4, W*4] shape (where M = 3 if
          `multimask_output=True` and M = 1 if `multimask_output=False`), the SAM
          output mask logits (before sigmoid) for the low-resolution masks, with 4x
          the resolution (1/4 stride) of the input backbone_features.
        - high_res_multimasks: [B, M, H*16, W*16] shape (where M = 3
          if `multimask_output=True` and M = 1 if `multimask_output=False`),
          upsampled from the low-resolution masks, with shape size as the image
          (stride is 1 pixel).
        - ious, [B, M] shape, where (where M = 3 if `multimask_output=True` and M = 1
          if `multimask_output=False`), the estimated IoU of each output mask.
        - low_res_masks: [B, 1, H*4, W*4] shape, the best mask in `low_res_multimasks`.
          If `multimask_output=True`, it's the mask with the highest IoU estimate.
          If `multimask_output=False`, it's the same as `low_res_multimasks`.
        - high_res_masks: [B, 1, H*16, W*16] shape, the best mask in `high_res_multimasks`.
          If `multimask_output=True`, it's the mask with the highest IoU estimate.
          If `multimask_output=False`, it's the same as `high_res_multimasks`.
        - obj_ptr: [B, C] shape, the object pointer vector for the output mask, extracted
          based on the output token from the SAM mask decoder.
        """
        B = backbone_features.size(0)
        device = backbone_features.device
        assert backbone_features.size(1) == self.sam_prompt_embed_dim
        assert backbone_features.size(2) == self.sam_image_embedding_size
        assert backbone_features.size(3) == self.sam_image_embedding_size

        # a) Handle point prompts
        if point_inputs is not None:
            sam_point_coords = point_inputs["point_coords"]
            sam_point_labels = point_inputs["point_labels"]
            assert sam_point_coords.size(0) == B and sam_point_labels.size(0) == B
        else:
            # If no points are provide, pad with an empty point (with label -1)
            sam_point_coords = torch.zeros(B, 1, 2, device=device)
            sam_point_labels = -torch.ones(B, 1, dtype=torch.int32, device=device)

        # b) Handle mask prompts
        if mask_inputs is not None:
            # If mask_inputs is provided, downsize it into low-res mask input if needed
            # and feed it as a dense mask prompt into the SAM mask encoder
            assert len(mask_inputs.shape) == 4 and mask_inputs.shape[:2] == (B, 1)
            if mask_inputs.shape[-2:] != self.sam_prompt_encoder.mask_input_size:
                sam_mask_prompt = F.interpolate(
                    mask_inputs.float(),
                    size=self.sam_prompt_encoder.mask_input_size,
                    align_corners=False,
                    mode="bilinear",
                    antialias=True,  # use antialias for downsampling
                )
            else:
                sam_mask_prompt = mask_inputs
        else:
            # Otherwise, simply feed None (and SAM's prompt encoder will add
            # a learned `no_mask_embed` to indicate no mask input in this case).
            sam_mask_prompt = None

        sparse_embeddings, dense_embeddings = self.sam_prompt_encoder(
            points=(sam_point_coords, sam_point_labels),
            boxes=None,
            masks=sam_mask_prompt,
        )

        ## Extension: LLM prompt
        if language_embd is not None:
            # B N C
            assert sparse_embeddings.size(0) == language_embd.size(0)
            assert sparse_embeddings.size(2) == language_embd.size(2)
            sparse_embeddings = torch.cat([sparse_embeddings, language_embd], dim=1)

        (
            low_res_multimasks,
            ious,
            sam_output_tokens,
            object_score_logits,
        ) = self.sam_mask_decoder(
            image_embeddings=backbone_features,
            image_pe=self.sam_prompt_encoder.get_dense_pe(),
            sparse_prompt_embeddings=sparse_embeddings,
            dense_prompt_embeddings=dense_embeddings,
            multimask_output=multimask_output,
            repeat_image=False,  # the image is already batched
            high_res_features=high_res_features,
        )
        if self.pred_obj_scores:
            is_obj_appearing = object_score_logits > 0

            # Mask used for spatial memories is always a *hard* choice between obj and no obj,
            # consistent with the actual mask prediction
            # print('Do torch.where !!!')
            # low_res_multimasks = torch.where(
            #     is_obj_appearing[:, None, None],
            #     low_res_multimasks,
            #     NO_OBJ_SCORE,
            # )

        # convert masks from possibly bfloat16 (or float16) to float32
        # (older PyTorch versions before 2.1 don't support `interpolate` on bf16)
        low_res_multimasks = low_res_multimasks.float()
        high_res_multimasks = F.interpolate(
            low_res_multimasks,
            size=(self.image_size, self.image_size),
            mode="bilinear",
            align_corners=False,
        )

        sam_output_token = sam_output_tokens[:, 0]
        if multimask_output:
            # take the best mask prediction (with the highest IoU estimation)
            best_iou_inds = torch.argmax(ious, dim=-1)
            batch_inds = torch.arange(B, device=device)
            low_res_masks = low_res_multimasks[batch_inds, best_iou_inds].unsqueeze(1)
            high_res_masks = high_res_multimasks[batch_inds, best_iou_inds].unsqueeze(1)
            if sam_output_tokens.size(1) > 1:
                sam_output_token = sam_output_tokens[batch_inds, best_iou_inds]
        else:
            low_res_masks, high_res_masks = low_res_multimasks, high_res_multimasks

        # Extract object pointer from the SAM output token (with occlusion handling)
        obj_ptr = self.obj_ptr_proj(sam_output_token)
        if self.pred_obj_scores:
            # Allow *soft* no obj ptr, unlike for masks
            if self.soft_no_obj_ptr:
                # Only hard possible with gt
                assert not self.teacher_force_obj_scores_for_mem
                lambda_is_obj_appearing = object_score_logits.sigmoid()
            else:
                lambda_is_obj_appearing = is_obj_appearing.float()

            if self.fixed_no_obj_ptr:
                obj_ptr = lambda_is_obj_appearing * obj_ptr
            obj_ptr = obj_ptr + (1 - lambda_is_obj_appearing) * self.no_obj_ptr

        return (
            low_res_multimasks,
            high_res_multimasks,
            ious,
            low_res_masks,
            high_res_masks,
            obj_ptr,
            object_score_logits,
        )
Forward SAM prompt encoders and mask heads.

Inputs:
- backbone_features: image features of [B, C, H, W] shape
- point_inputs: a dictionary with "point_coords" and "point_labels", where
1) "point_coords" has [B, P, 2] shape and float32 dtype and contains the
absolute pixel-unit coordinate in (x, y) format of the P input points
2) "point_labels" has shape [B, P] and int32 dtype, where 1 means
positive clicks, 0 means negative clicks, and -1 means padding
- mask_inputs: a mask of [B, 1, H*16, W*16] shape, float or bool, with the
same spatial size as the image.
- high_res_features: either 1) None or 2) or a list of length 2 containing
two feature maps of [B, C, 4*H, 4*W] and [B, C, 2*H, 2*W] shapes respectively,
which will be used as high-resolution feature maps for SAM decoder.
- multimask_output: if it's True, we output 3 candidate masks and their 3
corresponding IoU estimates, and if it's False, we output only 1 mask and
its corresponding IoU estimate.

Outputs:
- low_res_multimasks: [B, M, H*4, W*4] shape (where M = 3 if
`multimask_output=True` and M = 1 if `multimask_output=False`), the SAM
output mask logits (before sigmoid) for the low-resolution masks, with 4x
the resolution (1/4 stride) of the input backbone_features.
- high_res_multimasks: [B, M, H*16, W*16] shape (where M = 3
if `multimask_output=True` and M = 1 if `multimask_output=False`),
upsampled from the low-resolution masks, with shape size as the image
(stride is 1 pixel).
- ious, [B, M] shape, where (where M = 3 if `multimask_output=True` and M = 1
if `multimask_output=False`), the estimated IoU of each output mask.
- low_res_masks: [B, 1, H*4, W*4] shape, the best mask in `low_res_multimasks`.
If `multimask_output=True`, it's the mask with the highest IoU estimate.
If `multimask_output=False`, it's the same as `low_res_multimasks`.
- high_res_masks: [B, 1, H*16, W*16] shape, the best mask in `high_res_multimasks`.
If `multimask_output=True`, it's the mask with the highest IoU estimate.
If `multimask_output=False`, it's the same as `high_res_multimasks`.
- obj_ptr: [B, C] shape, the object pointer vector for the output mask, extracted
based on the output token from the SAM mask decoder.
projects/llava_sam2/hf/models/sam2.py