Support Sequence Packing with Context Parallel

[ahmadki]

We should have sequence packing support in the coming days (#300). This implementation doesn't support CP.

I'm opening this issue to start the discussion on how to support sequence packing with CP.

[SahilJain314]

Update: Sequence Packing support is merged. DTensor still does not support CP + Sequence packing. MCore does.

[snowmanwwg]

Closing this issue. Sequence packing + CP is supported, with a few exceptions that are tracked here
#718
#719
#769
#659