Fix/gemm rewriter bias add

[phager90]

Fixing #1068
Test will pass always due to an unfixed FIXME in check_op_count(..)

[TomWildenhain-Microsoft]

Also the rewrite_gemm should be moved to the end of the rewriters list (tfonnx.py line 454). Currently it is messing up a pattern that the LSTM rewriter looks for.

[phager90]

I moved the rewriter back as you suggested. I will check later the status of the unit tests.

BTW: what about the unit test that return true always? (see #1068)

[TomWildenhain-Microsoft]

Great! The graph validator should be fixed at some point but isn't actually that important. The unit testing system runs the converted graph through ONNX Runtime and compares the result with running the original through TF. The validator is just an additional check to make sure certain optimizations get made. If the unit test passes, we know conversion succeeds and gives the correct result.

[phager90]

Thanks for the clarifications!