[Refactor] extract block forward, backward and inference into a separate file

[justheuristic]

This PR does not change any functionality. It merely moves stuff around.

Problem: handler is huge and difficult to navigate. It is about to become more complicated once we support general peft

Suggesion: move all math (forward and backward pass; grad w.r.t. prompt, etc) to a separate file. Handler handles all networking and queues, but not the math; block_methods is responsible for math only.

List of changes:

• handler.py/_rpc_forward became block_methods/rpc_forward
• handler.py/_rpc_backward became block_methods/rpc_backward
• the math bits of rpc_inference were extracted into block_methods/iterate_rpc_inference

I took care to avoid any other changes

[borzunov]

Thanks for the PR.

Please rename it to block_functions.py and test that pushes are successfully sent (this is not covered by test and can break due to can_push refactor). Then it's good to go!