Modify mfsdp default data-parallel-sharding-strategy

[wplf]

PR for dev branch #3692

What does this PR do ?

The default value "no_shard" is not compatiable with many api. We can change this to optim_grads_params.

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[Phlip79]

Can you explain why you are making this change?

[wplf]

Can you explain why you are making this change?

Hi, I've found that when using no_shard, group.model_weight_buffer will not be created in

Megatron-LM/megatron/core/distributed/fsdp/src/megatron_fsdp/param_and_grad_buffer.py

Lines 2091 to 2109 in c5d8f6b

	if data_parallel_sharding_strategy != "no_shard":
	group.model_weight_buffer = DataParallelBuffer(
	self.ddp_config,
	group.params,
	is_data_distributed=is_model_weight_buffer_distributed
	and model_wbuf_dp_group.size() > 1,
	dtype=param_dtype,
	device=self.device,
	# Note: This will be DP-Outer + DP-Shard when sharding
	# the optimizer state in HFSDP, else just DP-Shard when
	# using basic HSDP or FSDP.
	data_parallel_group=model_wbuf_dp_group,
	is_transpose_buffer=False,
	temporary_bucket_allocator=self.weight_alloc,
	bucket_id=group_id,
	chunk_size_factor=group.chunk_size_factor,
	mem_alloc_context=self.mem_alloc_context,
	**main_buf_extra_kwargs,
	)

but somewhere will require group.mbuf, such as

1. swilgu's saving ckpt using fsdp_slice in

   Megatron-LM/megatron/core/transformer/fsdp_dtensor_checkpoint.py

   Line 234 in c5d8f6b

   fsdp_slice = dist_param.megatron_fsdp_slice

   , which is defined at

   Megatron-LM/megatron/core/distributed/fsdp/src/megatron_fsdp/param_and_grad_buffer.py

   Lines 2803 to 2806 in c5d8f6b

   	mbuf = pg.model_weight_buffer
   	if mbuf:
   	_start, _end = mbuf._get_item_slice_in_shard(item_id)
   	setattr(dist_param, "megatron_fsdp_slice", slice(_start, _end))

   . But mbuf is None, so this is the bug.
2. when using no_shard and no overlap_param_gather, code will step into else branch here

   Megatron-LM/megatron/core/distributed/fsdp/src/megatron_fsdp/megatron_fsdp.py

   Lines 1182 to 1203 in c5d8f6b

   	if not force_sync and self.ddp_config.overlap_param_gather:
   	# All-gather the first bucket before the forward pass.
   	if self.ddp_config.fsdp_all_gather_in_start_param_sync:
   	first_param = list(self.module.parameters())[0]
   	self.all_gather_and_wait_parameters_ready(
   	params=[first_param], prefetch=True, wait_bucket_ready=False
   	)
   	else:
   	self.synchronize_param_gather()
   	for bucket_id in range(self.all_gather_pipeline.num_buckets):
   	self.all_gather_pipeline.async_bucket_gather(bucket_id=bucket_id, bwd=False)
   	group = self.param_and_grad_buffer.parameter_groups[bucket_id]
   	if group.model_weight_buffer is None:
   	continue
   	if group.model_weight_buffer.is_data_distributed:
   	# If model weight is sharded, we wait for the all-gather to complete and
   	# then release the bucket immediately to save memory usage.
   	self.all_gather_pipeline.wait_bucket_ready(bucket_id, False)
   	for bucket_id in range(self.all_gather_pipeline.num_buckets):
   	self.all_gather_pipeline.wait_bucket_ready(bucket_id, False)

   . param.mbuf being None will raise bug

   Megatron-LM/megatron/core/distributed/fsdp/src/megatron_fsdp/param_and_grad_buffer.py

   Lines 4032 to 4038 in c5d8f6b

   	wbuf = self.get_fsdp_buffer(bucket_id, bwd)
   	# Lazy release the unused buckets.
   	self.recycle_unused_buckets()
   	# Allocate an empty bucket to store the module weights.
   	bucket = wbuf.fetch_bucket(set_param_data=True)

cc @shjwudp @cspades

[wplf]

For bug2, I think when mfsdp_strategy is "no_shard", we can skip param_gather like this, because optimizer is not sharded, and all params in local rank are valid. So we don't need gather from other ddp ranks any more.

The same logic is here

Megatron-LM/megatron/core/distributed/fsdp/src/megatron_fsdp/megatron_fsdp.py

Lines 434 to 435 in c5d8f6b

	if self.data_parallel_sharding_strategy == "no_shard":
	return

. Does this work for the condition?
cc @Phlip79 @cspades @shjwudp