Refactor the DeepSpeed strategy config management

[carmocca]

Outline & Motivation

DeepSpeed works by using a configuration file (dictionary) that allows customizing all of its aspects: https://www.deepspeed.ai/docs/config-json/

The DeepSpeedStrategy supports two ways of defining this:

1. Passing a config file, where every other argument becomes unused: https://github.com/Lightning-AI/lightning/blob/b792c90ea7148d61af192fde6c338ebbd355702f/src/lightning/fabric/strategies/deepspeed.py#L191
2. Exposes multiple of these arguments in the __init__ that are used to define a base config. https://github.com/Lightning-AI/lightning/blob/b792c90ea7148d61af192fde6c338ebbd355702f/src/lightning/fabric/strategies/deepspeed.py#L242-L271

Option 2 is not scalable because:

• It forces us to duplicate all arguments
• Our docstrings might become outdated
• Our strategy defaults might diverge from the defaults in deepspeed
• It forces the user to either create an entire config or use these arguments
• Arguments might be different based on the installed deepspeed version as we support more than a single version.
• When deepspeed adds an argument that we don't expose, users have to switch to using the config

Pitch

Remove all these exposed arguments and just have a config argument that overloads support for:

• Passing a path to a config file

  DeepSpeedStrategy(config="my/config/path.json")

• Passing a full config object

  config = ds.runtime.config.DeepSpeedConfig({"train_micro_batch_size_per_gpu": 2})
  DeepSpeedStrategy(config=config)

• Passing a config dictionary (or a subset of it) that will update the default config

  config = {"zero_optimization": {"offload_optimizer": {"device": "cpu"}}}
  DeepSpeedStrategy(config=config)

Where the default config is created by calling: https://github.com/microsoft/DeepSpeed/blob/085981bf1caf5d7d0b26d05f7c7e9487e1b35190/deepspeed/runtime/config.py#L674

Additional context

DeepSpeed is considered experimental so we could do this breaking change: https://github.com/Lightning-AI/lightning/blob/b792c90ea7148d61af192fde6c338ebbd355702f/src/lightning/fabric/strategies/deepspeed.py#L99

cc @justusschock @awaelchli @carmocca

[keunwoochoi]

+1 for this!