Support MatMul operator when loading onnx

[kindsenior]

Nice to send you a Pull Request.
I'm Kunio Kojima at the University of Tokyo and a beginner of nngen.

When we loaded a network from an onnx file like nngen.from_onnx(filename),
we got the follow.

/usr/local/lib/python3.7/dist-packages/nngen-1.3.0-py3.7.egg/nngen/onnx/init.py in _get_func(op_type)
def _get_func(op_type)
--->return func_map[op_type]
KeyError: 'Mat'

It seems that nngen supports MatMul (nngen.operator.matmul).
However, it does not support loading MatMul operator from onnx file.

nngen/nngen/onnx/__init__.py

Lines 36 to 71 in cba265b

	func_map = {
	'Add': basic.Add,
	'Sub': basic.Sub,
	'Mul': basic.Mul,
	'Div': basic.Div,
	'Exp': exp.Exp,
	'ReduceSum': reduce.ReduceSum,
	'ReduceMax': reduce.ReduceMax,
	'ReduceMin': reduce.ReduceMin,
	'ArgMax': reduce.ArgMax,
	'ArgMin': reduce.ArgMin,
	'Conv': conv.Conv,
	'Gemm': gemm.Gemm,
	'AveragePool': pool.AveragePool,
	'GlobalAveragePool': pool.GlobalAveragePool,
	'MaxPool': pool.MaxPool,
	'Pad': pad.Pad,
	'Relu': act_func.Relu,
	'LeakyRelu': act_func.LeakyRelu,
	'Sigmoid': act_func.Sigmoid,
	'BatchNormalization': batchnormalization.BatchNormalization,
	'Shape': shape.Shape,
	'Reshape': reshape.Reshape,
	'Flatten': flatten.Flatten,
	'Upsample': upsample.Upsample,
	'Transpose': transpose.Transpose,
	'Concat': concat.Concat,
	'Squeeze': squeeze.Squeeze,
	'Unsqueeze': squeeze.Unsqueeze,
	'Gather': gather.Gather,
	'Slice': slice_.Slice,
	'Cast': cast.Cast,
	'Ceil': ceil.Ceil,
	'Floor': floor.Floor,
	'Identity': identity.Identity,
	}

Therefore, we added a mapping from onnx's MatMul to nngen.operator.matmul.

[kindsenior]

Sorry.
I have misunderstood the operator.basic.
I understand it is the operator for element-wise procedures.
Therefore, my implementation is wrong.