Clean up dead code and recyle tensor when it is out of scope. (#137)

Summary:
Pull Request resolved: #137

Clean up dead code:

    removed two not used functions: analyze_ops and analyze_subgraph
    cleaned up other code related to these two functions.

Recycled tensor when it is no longer live: recorded the last op id that refers a tensor id, when replay passing this last op id, recycled the tensor.

With this fix, CMF V0 model can now be replayed. It was out of memory before.

Reviewed By: briancoutinho

Differential Revision: D59937145

fbshipit-source-id: f4b3ff447b09e2b0f2c57863bd84167a37180894