[PyTorchEdge] Export flatbuffers from _save_parameters() (#74579)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/74579
Now that we can convert a module to a flatbuffer, update `_save_parameters()` to optionally write to that format.
Also, rename the internal `ScriptModuleSerializer` class to `IValuePickler` to make it more clear that a) it's pickle-specific, and b) it serializes IValues, not Modules.
ghstack-source-id: 152487889
Test Plan:
New unit test shows that we can produce Flatbuffer-formatted output.
```
$ buck test //xplat/caffe2:test_lite_trainer //xplat/caffe2:test_lite_trainer_pickle_and_flatbuffer
Building: finished in 0.5 sec (100%) 346/346 jobs, 0/346 updated
Total time: 0.6 sec
Testing: finished in 0.5 sec (26 PASS/0 FAIL)
BUILD SUCCEEDED
RESULTS FOR //xplat/caffe2:test_lite_trainer //xplat/caffe2:test_lite_trainer_pickle_and_flatbuffer
PASS <100ms 13 Passed 0 Skipped 0 Failed //xplat/caffe2:test_lite_trainer
PASS <100ms 13 Passed 0 Skipped 0 Failed //xplat/caffe2:test_lite_trainer_pickle_and_flatbuffer
TESTS PASSED
```
A new test in later commit D34488913 tests the full round trip.
Reviewed By: qihqi
Differential Revision: D34408538
fbshipit-source-id: eea183c31b5e1b2b75a65f384d8a479223a4ae72
(cherry picked from commit de310a15422b65fb7e443f7005d287d9f5f586bc)
