[fbcode][pytorch mobile] Create model reader utilities.

Summary:
For some of the end to end flow projects, we will need the capabilities to read module information during model validation or model publishing.
Creating this model_reader.py for utilities for model content reading, this diff we included the following functionalities:
1. read the model bytecode version;
2. check if a model is lite PyTorch script module;
3. check if a model is PyTorch script module.

This diff is recreated from the reverted diff: D24655999 (7f056e9).

Test Plan:
```
[xcheng16@devvm1099]/data/users/xcheng16/fbsource/fbcode% buck test //caffe2/torch/fb/mobile/tests:mobile_model_reader_tests
Action graph will be rebuilt because files have been added or removed.
Parsing buck files: finished in 10.4 sec
Creating action graph: finished in 22.2 sec
Building: finished in 01:29.1 min (100%) 10619/10619 jobs, 1145 updated
  Total time: 02:01.8 min
More details at https://www.internalfb.com/intern/buck/build/f962dfad-76f9-457a-aca3-768ce20f0c31
Tpx test run coordinator for Facebook. See https://fburl.com/tpx for details.
Running with tpx session id: 172633f6-6b5b-49e9-a632-b4efa083a001
Trace available for this run at /tmp/tpx-20201109-165156.109798/trace.log
Started reporting to test run: https://our.intern.facebook.com/intern/testinfra/testrun/3940649712677511
    ✓ ListingSuccess: caffe2/torch/fb/mobile/tests:mobile_model_reader_tests - main (18.229)
    ✓ Pass: caffe2/torch/fb/mobile/tests:mobile_model_reader_tests - test_is_pytorch_lite_module (caffe2.torch.fb.mobile.tests.test_model_reader.TestModelLoader) (8.975)
    ✓ Pass: caffe2/torch/fb/mobile/tests:mobile_model_reader_tests - test_is_pytorch_script_module (caffe2.torch.fb.mobile.tests.test_model_reader.TestModelLoader) (9.136)
    ✓ Pass: caffe2/torch/fb/mobile/tests:mobile_model_reader_tests - test_read_module_bytecode_version (caffe2.torch.fb.mobile.tests.test_model_reader.TestModelLoader) (9.152)
Summary
  Pass: 3
  ListingSuccess: 1
Finished test run: https://our.intern.facebook.com/intern/testinfra/testrun/3940649712677511
```

Reviewed By: husthyc

Differential Revision: D24848563

fbshipit-source-id: ab3371e111206a4bb4d07715c3314596cdc38d2c

torch/utils/show_pickle.py

@@ -68,6 +68,7 @@ def persistent_load(self, pid):
     def dump(cls, in_stream, out_stream):
         value = cls(in_stream).load()
         pprint.pprint(value, stream=out_stream)
+        return value
 
 
 def main(argv, output_stream=None):