[PyTorch Mobile] Preserve bundled input related methods when calling optimize_for_mobile
Summary:
Added an extra step to **always** preserve the bundled inputs methods if they are present in the input module.
Also added a check to see if all the methods in the `preseved_methods` exist. If not, we will now throw an exception. This can hopefully stop hard-to-debug inputs from getting into downstream functions.
~~Add an optional argument `preserve_bundled_inputs_methods=False` to the `optimize_for_mobile` function. If set to be True, the function will now add three additional functions related with bundled inputs to be preserved: `get_all_bundled_inputs`, `get_num_bundled_inputs` and `run_on_bundled_input`.~~
Test Plan:
`buck test mode/dev //caffe2/test:mobile -- 'test_preserve_bundled_inputs_methods \(test_mobile_optimizer\.TestOptimizer\)'`
or
`buck test caffe2/test:mobile` to run some other related tests as well.
Reviewed By: dhruvbird
Differential Revision: D25433268
fbshipit-source-id: 0bf9b4afe64b79ed1684a3db4c0baea40ed3cdd5
