[fbcode] Make 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.

Test Plan:
```
[xcheng16@devvm1099]/data/users/xcheng16/fbsource/fbcode% buck test pytorch_mobile/utils/tests:mobile_model_reader_tests
Processing filesystem changes: finished in 1.5 sec
Parsing buck files: finished in 1.6 sec
Building: finished in 4.9 sec (100%) 9249/43504 jobs, 2 updated
  Total time: 6.5 sec
More details at https://www.internalfb.com/intern/buck/build/6d0e2c23-d86d-4248-811f-31cb1aa7eab3
Tpx test run coordinator for Facebook. See https://fburl.com/tpx for details.
Running with tpx session id: 2ffccd62-ece5-44b5-8350-3a292243fad9
Trace available for this run at /tmp/tpx-20201030-122220.664763/trace.log
Started reporting to test run: https://our.intern.facebook.com/intern/testinfra/testrun/3940649711969390
    ✓ ListingSuccess: pytorch_mobile/utils/tests:mobile_model_reader_tests - main (10.234)
    ✓ Pass: pytorch_mobile/utils/tests:mobile_model_reader_tests - test_is_pytorch_lite_module (pytorch_mobile.utils.tests.test_model_reader.TestModelLoader) (7.039)
    ✓ Pass: pytorch_mobile/utils/tests:mobile_model_reader_tests - test_is_pytorch_script_module (pytorch_mobile.utils.tests.test_model_reader.TestModelLoader) (7.205)
    ✓ Pass: pytorch_mobile/utils/tests:mobile_model_reader_tests - test_read_module_bytecode_version (pytorch_mobile.utils.tests.test_model_reader.TestModelLoader) (7.223)
Summary
  Pass: 3
  ListingSuccess: 1
Finished test run: https://our.intern.facebook.com/intern/testinfra/testrun/3940649711969390

Reviewed By: husthyc

Differential Revision: D24655999

fbshipit-source-id: 5095ca158d89231fb17285d445548f91ddb89bab

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):