[platform] support pytorch custom op pluggable

[wangxiyuan]

Allow register platform specified forward function by users.

Related to: #11162

How to use:

In an out-of-tree plugin repo(for example, the npu device):

from vllm.model_executor.layers.layernorm import RMSNorm

def forward_npu(self, x, residual):
     # implementation
     ...

RMSNorm.set_foward_method(forward_npu)

[github-actions]

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