fix zero arg export in training_ir and constant tensor handling (#4382)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4382

Prepare for the re-land of D60006710.

Previously, some buffers are not correctly identified. D60006710 fixes it but causes test failures. This pr did a patch to the test to avoid test failures.

Reviewed By: BoyuanFeng

Differential Revision: D60137883

fbshipit-source-id: d919e25525347a86afc6a895d2b0eb94d161b5ad