[PyTorch][Edge] Improve InflatableArgs for Bundled Inputs (#62368)

[PyTorch][Edge] Improve InflatableArgs for Bundled Inputs (#62368)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/62368

# Context
The bundled inputs accepts an expression in the form of string InflatableArg.fmt that can be applied on the inputs to inflate. The InflatableArg.fmt provides flexibility to have custom transformation to inflate. When the input arguments to a function are not Tensor type, TorchScript casts the inputs from type T to Optional[T] expects the function to handle Nullable (None) clause as well. This becomes tricky to handle in one line code or lambda functions.

We propose an alternative way which allows InflatableArg to include the text of a TorchScript function that would be defined on the module as a helper, then use that in its inflation expression. This can be provided by InflatableArg.fmt_fn. Please refer to pytorch/test/test_bundled_inputs.py for example on how to use the same.

Also refer JacobSzwejbka comment on the same [here](https://github.com/pytorch/pytorch/pull/62368#issuecomment-892012812)

# Mitigation
Allow InflatedArg to include the text of a TorchScript function that would be defined on the module as a helper, then use that in its inflation expression.
ghstack-source-id: 135158680

Test Plan:
To run `test_dict_args`

```
(base) [pavithran@devvm1803.vll0 /data/users/pavithran/fbsource/fbcode] buck test //caffe2/test:test_bundled_inputs -- test_dict_args
Action graph will be rebuilt because files have been added or removed.
Building: finished in 5.4 sec (100%) 12180/12180 jobs, 0/12180 updated
  Total time: 5.8 sec
More details at https://www.internalfb.com/intern/buck/build/fafcf277-1095-4cba-978d-6022f0d391ad
Tpx test run coordinator for Facebook. See https://fburl.com/tpx for details.
Running with tpx session id: 5ef9de71-c1b1-406b-a6c0-3321c2368b8d
Trace available for this run at /tmp/tpx-20210727-163946.454212/trace.log
Started reporting to test run: https://www.internalfb.com/intern/testinfra/testrun/7036874465805934
    ✓ ListingSuccess: caffe2/test:test_bundled_inputs - main (11.365)
    ✓ Pass: caffe2/test:test_bundled_inputs - test_dict_args (test_bundled_inputs.TestBundledInputs) (12.307)
Summary
  Pass: 1
  ListingSuccess: 1
If you need help understanding your runs, please follow the wiki: https://fburl.com/posting_in_tpx_users
Finished test run: https://www.internalfb.com/intern/testinfra/testrun/7036874465805934
```

To check the py code of TS module:
P433043973

Reviewed By: dreiss

Differential Revision: D29950421

fbshipit-source-id: c819ec5c94429b7fbf6c4beb0259457f169b08ec