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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def cast(attrs, inputs, proto_obj):
 """ Cast input to a given dtype"""

    try:
        from onnx.mapping import TENSOR_TYPE_TO_NP_TYPE
    except ImportError:
        raise ImportError("Onnx and protobuf need to be installed. "
                          + "Instructions to install - https://github.com/onnx/onnx")
    new_attrs = translation_utils._fix_attribute_names(attrs, {'to' : 'dtype'})
    new_attrs['dtype'] = TENSOR_TYPE_TO_NP_TYPE[int(new_attrs['dtype'])]
    return 'cast', new_attrs, inputs