ZeroBubble Pipeline Parallelism

[pgusto34]

High-level Description

Added split backward pass support to Piper, which enabled the system to support a wider array of schedules (e.g., ZeroBubble).

Description of changes:

• Refactored piper into a piper directory/project and renamed files accordingly by removing the "piper" prefix (if you prefer to keep src as the directory name, that's totally cool too)
• Added the backward_weight() and backward_input() functions to actor.py, which are the functions that implement the split backward pass
• Added a backward_utils file containing helper functions for the split_backward_pass. These functions are essentially identically to the ones that PiPPy uses outside of a few minor tweaks.
• Updated the Task class in exec.py to store a TaskType enum instead of booleans when determining what type of task type (f, b, u, i, w) each object represents
• Updated the validate_schedule() function in exec.py to use the updated Task class and check for correctness when backward_input/backward_weight tasks could be present.
• Updated piper_exec() in exec.py to include cases for handling backward_input and backward_weight tasks
• Added zb1p as an option to the test_llama.py --schedule argument
• Modified the schedule_helpers.py to use the updated Task class
• Created a Dockerfile and run.sh script to deploy Piper in a Docker container