Update Explicit __init__ for SDK and propagate import paths (#1151)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1151

Based on the design linked below, this diff updates the `__init__.py` of SDK and Inspector to provide clear import paths for users

In addition, it renames `inspector.py` -> `_inspector.py` to deter manual access
  - With exception of tests

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Note:
- Outside of the ETRecord folders, no one should be using `from executorch.sdk.etrecord import ...`
- Updating SDK file names to deter import ("_") will be done in separate diffs

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Design: https://docs.google.com/document/d/1ugPAPmKzc0oOeL9Vjssgx1UvQP6KxdDJK06wFRHtgAU/edit

Reviewed By: tarun292

Differential Revision: D50943994

fbshipit-source-id: 79b7e17be72d9166da86cce9f4183e94ccc2c8b4