[Frontend][TF] Fix Placeholder issue

[yongwww]

This pr is a fix for issue "Math op expect 2 inputs, 1 given" in tf frontend. This frozen pb in the repo can be used to reproduce this issue. The placeholder node is at the end of the graphdef for some reason, so we need to preprocess these ops during first traversal before they are used.
@srkreddy1238 @wweic @kazum please take a look.

[kazum]

@yongwww Can you add a small regression test to tests/python/frontend/tensorflow/test_forward.py to prevent the problem from happening again?

[yongwww]

@kazum sure, I'll add a test case with a small code snippet or a small .pb graphdef

[srkreddy1238]

@yongwww
In general tensorflow export happens by optimisation which internally does a topological ordering. Not sure how this graph was handled.

Anyway this change is good to have.

[yongwww]

@srkreddy1238 yeah, the graphdef is kind of weird, but it should be valid, probably caused by ProtocolBuffer. We also met this issue when handling control flow stuff since there are cycles in graph, which makes topological ordering behave abnormally.

[kazum]

@yongwww, can you fix the CI error?

[yongwww]

@kazum I am working on it, will update soon

[yongwww]

@kazum test case should be okay, works on my local env. In order to pass CI, need to get the pb file (dmlc/web-data#172) uploaded first, could you please take a look at the PRs

[kazum]

Thanks @yongwww @srkreddy1238. This is now merged.