fix dh0 is None breaking backward pass

[Sxela]

Originally when running backward pass I get:

process_batch
    accelerator.backward(loss)
  File "/usr/local/lib/python3.10/dist-packages/accelerate/accelerator.py", line 1983, in backward
    self.scaler.scale(loss).backward(**kwargs)
  File "/usr/local/lib/python3.10/dist-packages/torch/_tensor.py", line 581, in backward
    torch.autograd.backward(
  File "/usr/local/lib/python3.10/dist-packages/torch/autograd/__init__.py", line 347, in backward
    _engine_run_backward(
  File "/usr/local/lib/python3.10/dist-packages/torch/autograd/graph.py", line 825, in _engine_run_backward
    return Variable._execution_engine.run_backward(  # Calls into the C++ engine to run the backward pass
  File "/usr/local/lib/python3.10/dist-packages/torch/autograd/function.py", line 307, in apply
    return user_fn(self, *args)
  File "/usr/local/lib/python3.10/dist-packages/fla/utils.py", line 18, in wrapper
    return fn(ctx,
  File "/usr/local/lib/python3.10/dist-packages/torch/amp/autocast_mode.py", line 511, in decorate_bwd
    return bwd(*args, **kwargs)
  File "/usr/local/lib/python3.10/dist-packages/fla/ops/rwkv6/fused_recurrent.py", line 601, in backward
    return dq.to(q), dk.to(k), dv.to(v), dw.to(w), du.to(u), None, dh0.to(initial_state), None, None, None, None
AttributeError: 'NoneType' object has no attribute 'to'

Steps to reproduce:

import torch
import torch.nn as nn
from fla.layers.rwkv6 import RWKV6Attention

layer = RWKV6Attention(hidden_size=320).cuda()
t = torch.randn((300,10,320)).cuda()
t.requires_grad = True

t2 = torch.randn((300,10,320)).cuda()
res = layer(t)[0]

criterion = nn.MSELoss()

loss = criterion(t2, res)
loss.backward()

[yzhangcs]

Thank you!