Add Iteration related Events

[b-chu]

What does this PR do?

Adds ITERATION_START, ITERATION_END, and ITERATION_CHECKPOINT to Event, Engine, and Callback. Increments iteration during training based on iteration length defined in State. Iteration length is a private variable in State and should have no effect by default.

commit-id:bdbe33f2

---

Stack:

• Add Iteration related Events to callbacks #3087
• Add Iteration related Events #3076 ⬅

⚠️ Part of a stack created by spr. Do not merge manually using the UI - doing so may have unexpected results.

What issue(s) does this change relate to?

Before submitting

• Have you read the contributor guidelines?
• Is this change a documentation change or typo fix? If so, skip the rest of this checklist.
• Was this change discussed/approved in a GitHub issue first? It is much more likely to be merged if so.
• Did you update any related docs and document your change?
• Did you update any related tests and add any new tests related to your change? (see testing)
• Did you run the tests locally to make sure they pass?
• Did you run pre-commit on your change? (see the pre-commit section of prerequisites)

[mvpatel2000]

Can you please add some tests that run multiple iterations? Ideally we have a test that also tries changing iteration length and tests new API in state

[bcui19]

From my perspective, this seems good. Added some comments, but yea it'd be good to add more testing/also include tests on if we have early breaks from an epoch in an iteration.

[b-chu]

For early breaks in iteration, this would be implemented by users via callback so wouldn't be tested here