Retiarii graph and code generation test

[ultmaster]

Sorted in priority from high to low.

• Support customized call method. (for example self.encode in self.forward)
• Support operator prim::ListUnpack.
• If condition: self.upsample
• Support torchvision models.
• Complex argument list in snli.

I've bypassed the corresponding tests for now.

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Workarounded issues:

• id is not reliable for records as it an be reused when objects have been destroyed.
• records should be cleared when the whole procedure is completed.

Question: Are records actually needed in most cases?

UPD: records have been refactored in another PR.