fix model error

[f-hy]

fix the following error:

Traceback (most recent call last):
  File "e:\data\frid\python\codes\adlpi\labml_nn\transformers\rope\__init__.py", line 231, in <module>
    _test_rotary()
  File "e:\data\frid\python\codes\adlpi\labml_nn\transformers\rope\__init__.py", line 227, in _test_rotary
    inspect(rotary_pe(x))
  File "E:\data\frid\python\p121\lib\site-packages\torch\nn\modules\module.py", line 1518, in _wrapped_call_impl
  File "e:\data\frid\python\codes\adlpi\labml_nn\transformers\rope\__init__.py", line 188, in forward
    x_rope = (x_rope * self.cos_cached[:x.shape[0]]) + (neg_half_x * self.sin_cached[:x.shape[0]])
RuntimeError: The size of tensor a (3) must match the size of tensor b (4) at non-singleton dimension 3

the reason is that the dimensions of two multiplied vectors do not match.

[vpj]

In cached sin and cos the first dimension is the sequence length. So it should be cos_cached[:x.shape[0]]

[chen-xin-94]

In cached sin and cos the first dimension is the sequence length. So it should be cos_cached[:x.shape[0]]

That's right. I think the fix should be like this: #249