Make DDP/FSDP a regular transform

[t-vi]

🚀 Feature

Make DDP/FSDP a regular transform (to a large part including making transforms flexible enough to support this).

Motivation

Currently DDP/FSDP is not a regular transform, leading to things like #94 and limiting composability / sequencing.
One of the key bits is that DDP/FSDP would need to do the adjustments we currently do to the prologue during tracing with DDP/FSDP in the transform, so we need to allow mutation of prologues through transforms. This is also in line with similar needs for other transforms (lora, quantization, but also value-and-grad-things) that change prologue signatures, so this generalization should happen.

cc @carmocca @awaelchli @crcrpar

[IvanYashchuk]

What is meant by making DDP/FSDP a regular transform? What are you planning to do?
Today it's not a transform at all, as I commented here #94 (comment). thunder.distributed.ddp/fsdp only annotate parameters for tracing. It's also described in the tutorial https://github.com/Lightning-AI/lightning-thunder/blob/main/notebooks/dev_tutorials/fsdp_tutorial.ipynb

I don't see any other way for sharding happen somewhen after the thunder.jit(model) call. What ideas do you have?
The current workflow is

1. Shard the model
   Done with thunder.distributed.fsdp(model) or with torch.distributed.FullyShardedDataParallel in PyTorch
2. Set up the optimizer using the sharded model so that the optimizer state is a shard
3. Call thunder.jit(sharded_model) or torch.compile(sharded_model) in PyTorch.

[t-vi]

I would like to move 3 up (for thunder.jit).

[IvanYashchuk]

Is the preferred order then 3 -> 1 -> 2?

[t-vi]

So per discussions with @crcrpar and @IvanYashchuk (thank you!)

• This needs to modify model & prologue and compute trace (and invalidate / modify old cached entries).
• Needs to come before autograd and stay compatible.
• We need a good way to represent the changes to model state, the current goal is to put this on the ThunderModule and leave the user modules intact. This is why Change prologue details to prepare for fsdp as a transform #228 simplifies parameter access and also we want to access the thunder module in the prologue (which has also come up before).

(obviously good ideas from Masaki and Ivan, not so good ones my own)

[mruberry]

triage review — let's start design review with draft PR to discuss