Add to_dataframe for Inspector and Event (#1476)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1476

Resolve #TODO executorch/sdk/inspector/_inspector.py lines=555:
```
# TODO: push row generation down to Event
```
Add `Inspector.to_dataframe()` API to get the composite dataframe of all `EventBlocks`

Update docs accordingly

As discussed with Olivia-liu, it would be ineffective to store timescale data in each `Event`. When calling `Event.to_dataframe`, we don't directly present the option to set the unit since this is mostly intended to make things easier when operating from the `EventBlock` layer of abstraction and higher.

Reviewed By: Jack-Khuu

Differential Revision: D52372928

fbshipit-source-id: cbff3df21912c90121b78c1d07f2174e66ca6aa1