Cherry-pick Megatron-LM's changes in pipeline model parallel for T5 (N…

…VIDIA#1232)

* update parallel_state

* update pipeline common funcs - forward_step and backward_step

* update pipelining w/o interleaving

* type hint

* merge utils into without_interleaving

Motivation: functions in utils are only used by
forward_backward_pipelining_without_interleaving

* fix handling of `model_type`

* fix import of DDP

* update set_input_tensor method

* fix

* cosmetic

* update model

* refactor pipeline test scripts