Update on "[ET-VK] 4/n Split dispatches between multiple command buffers. Removing flush function call from submit_current_cmd_and_wait."

This diff removes the `flush` function call from `submit_current_cmd_and_wait` and instead adds it to the `encode_prepack` method.

This change will allow better control over flush which is needed in follow up diffs.

Differential Revision: [D78360036](https://our.internmc.facebook.com/intern/diff/D78360036/)

[ghstack-poisoned]

Author: trivedivivek