[PyTorchEdge] Make _load_parameters() handle flatbuffer inputs (#74580)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/74580
Handle Flatbuffer-serialized parameters.
Make `_load_parameters()` detect the input data format and use the correct deserializer to load the parameters.
Also, rename `BytecodeDeserializer` to `IValueUnpickler` to make it clear that it unpickles an `IValue` and doesn't have anything to do with bytecode.
ghstack-source-id: 152487890
Test Plan:
New unit test shows a successful round trip from _save_parameters() to _load_parameters() using flatbuffers.
```
$ 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
```
Reviewed By: qihqi
Differential Revision: D34488913
fbshipit-source-id: 8d2c0b895699f3b336115d33bf96d49cbf9245d2
(cherry picked from commit 319345deff260826197f8cdf5ac03071b412c72f)
