[PyTorchEdge] Add getFileFormat() so we can differentiate Zip/Pickle from Flatbuffer (#73707)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/73707
Add a helper function to detect the file format from the first bytes of a data file or stream. This will be necessary during the migration from Pickle-serialized modules to Flatbuffer-serialized modules.
ghstack-source-id: 150384317
Test Plan:
Existing tests for ZIP+Pickle continue to pass.
New unit tests pass:
```
cd xplat && buck test //xplat/caffe2:test_lite_trainer //xplat/caffe2:test_lite_interpreter
Building: finished in 26.6 sec (100%) 3180/3180 jobs, 571/3180 updated
Total time: 32.2 sec
Testing: finished in 07:08.3 min (89 PASS/0 FAIL)
BUILD SUCCEEDED
RESULTS FOR //xplat/caffe2:test_lite_interpreter //xplat/caffe2:test_lite_trainer
PASS 421.1s 81 Passed 0 Skipped 0 Failed //xplat/caffe2:test_lite_interpreter
PASS 103ms 8 Passed 0 Skipped 0 Failed //xplat/caffe2:test_lite_trainer
TESTS PASSED
```
Reviewed By: iseeyuan
Differential Revision: D34527859
fbshipit-source-id: ff2d1eabc2f8be1de2e44709c878e2d1a373f0df
(cherry picked from commit 5c394848346ab9e374c9e7eed479ad70ed09a7ae)
