Fix training of pipeline based peft's lora model

[xuanhua]

Hi, guys

I find there is an assert failure when I train huggingface's lora based model in pipeline style.

Here is the whole steps that I created my model:

1. Load the pre-trained chatglm-6b model from huggingface, as Model_A
2. Use huggingface's peft's get_peft_model(...) and my LoraConfig(...) from Model_A to create the lora model, as Model_B
3. Create my own pipeline based model Model_C from Model_B

And I run Model_C under 2 3090ti GPUs. And the assertion failure looks like this:

Traceback (most recent call last):
  File "/home/ubuntu/proj/chatglm-finetuning/train_pipeline.py", line 372, in <module>
    main()
  File "/home/ubuntu/proj/chatglm-finetuning/train_pipeline.py", line 351, in main
    loss = engine.train_batch(data_iter=train_dataloader)
  File "/home/ubuntu/anaconda3/lib/python3.9/site-packages/deepspeed/runtime/pipe/engine.py", line 375, in train_batch
    self._exec_schedule(sched)
  File "/home/ubuntu/anaconda3/lib/python3.9/site-packages/deepspeed/runtime/pipe/engine.py", line 1375, in _exec_schedule
    self._exec_instr(**cmd.kwargs)
  File "/home/ubuntu/anaconda3/lib/python3.9/site-packages/deepspeed/runtime/pipe/engine.py", line 276, in _exec_reduce_tied_grads
    dist.all_reduce(grad, group=group)
  File "/home/ubuntu/anaconda3/lib/python3.9/site-packages/deepspeed/comm/comm.py", line 117, in log_wrapper
    return func(*args, **kwargs)
  File "/home/ubuntu/anaconda3/lib/python3.9/site-packages/deepspeed/comm/comm.py", line 496, in all_reduce
    return cdb.all_reduce(tensor, op, group, async_op)
  File "/home/ubuntu/anaconda3/lib/python3.9/site-packages/deepspeed/comm/torch.py", line 159, in all_reduce
    return torch.distributed.all_reduce(tensor=tensor, op=op, group=group, async_op=async_op)
  File "/home/ubuntu/anaconda3/lib/python3.9/site-packages/torch/distributed/distributed_c10d.py", line 1520, in all_reduce
    _check_single_tensor(tensor, "tensor")
  File "/home/ubuntu/anaconda3/lib/python3.9/site-packages/torch/distributed/distributed_c10d.py", line 463, in _check_single_tensor
    raise RuntimeError(
RuntimeError: Invalid function argument. Expected parameter `tensor` to be of type torch.Tensor.

After some debugging, I find out the root cause is that my configuration of lora (in below) only add extra lora layer(part) in qkv related layers but not the embedding layer. So the whole embedding layer's parameters are freezed.

lora_config = LoraConfig(r=8, # copied from finetuning_lora.py
                        lora_alpha=32,
                        target_modules=["query_key_value"],
                        lora_dropout=0.1,
                        bias="none",
                        task_type="CAUSAL_LM",
                        inference_mode=False,
                        )   

And in my implementation of pipeline based model, I declared the embeding layer as a tied-layer. So the whole thing is that there are no gradients at all for embedding layer, but embedding layer as the tied layer needs to be synced between two gpus. The value of gradient is None but is still passed to all_reduce operation.

Current, my fix is simple and add a check if this grad is None.

[xuanhua]

@duli2012 Hi, I'm not sure if this pull request meet the project's requirement ? Or any suggestions on this PR, expect your reply :)