    def slice(self, embedding, padding_mask, attn_mask, sampling_factor):
        """
        embedding is a (T, B, D) tensor
        padding_mask is a (B, T) tensor or None
        attn_mask is a (T, T) tensor or None
        """
        embedding = embedding[::sampling_factor, :, :]
        if padding_mask is not None:
            padding_mask = padding_mask[:, ::sampling_factor]
        if attn_mask is not None:
            attn_mask = attn_mask[::sampling_factor, ::sampling_factor]

        return embedding, padding_mask, attn_mask
embedding is a (T, B, D) tensor
padding_mask is a (B, T) tensor or None
attn_mask is a (T, T) tensor or None
examples/speech_recognition/models/vggtransformer.py