Returns indices of the minimum values along an axis.

def argmin(attrs, inputs, proto_obj):
    """Returns indices of the minimum values along an axis."""
    axis = attrs.get('axis', 0)
    keepdims = attrs.get('keepdims', 1)
    argmin_op = symbol.argmin(inputs[0], axis=axis, keepdims=keepdims)
    # onnx argmax operator always expects int64 as output type
    cast_attrs = {'dtype': 'int64'}
    return 'cast', cast_attrs, argmin_op