Fix #920: Check cross entropy monkey patch

[0xtoward]

Summary

Fix #920
Note: The test uses subprocess isolation because cross_entropy patches modify a global function (transformers.loss.loss_utils.nn.functional.cross_entropy). I'm wondering if there is an elegant way.

Testing Done

• Hardware Type:
• run make test to ensure correctness
• run make checkstyle to ensure code style
• run make test-convergence to ensure convergence

[Tcc0403]

I'm thinking about just patching the forward method as flce does with some modifcitions in liger's loss_utils, so we only need to toggle an option to switch between flce and ce with functools.partial. What do you think?