add comment on how to generate onnx and tf files (#748)

Co-authored-by: mvermeulen <5479696+mvermeulen@users.noreply.github.com>

test/onnx/gen_onnx.py

+# This script generates onnx files for MIGraphX onnx operator tests.
+# To generate an individual onnx file, you can use the following
+# command: python -c "import gen_onnx; gen_onnx.{test_name}_test()"
 import numpy as np
 import onnx
 from onnx import helper

test/tf/gen_tf_pb.py

+# This script generates tf pb files for MIGraphX tf operator tests.
+# To generate an individual pb file, you can use the following
+# command: python -c "import gen_tf_pb; gen_tf_pb.{test_name}_test()"
 import numpy as np
 import tensorflow as tf
 from tensorflow.core.framework import attr_value_pb2