ciflow/xpu/185531: Add oneDNN LSTM primitive support for XPU inference

Use dnnl::lstm_forward primitive on XPU for LSTM inference, providing

significant speedup over the per-timestep fused cell approach by

eliminating kernel launch overhead entirely.

Key changes:

• New aten/src/ATen/native/mkldnn/xpu/RNN.cpp implementing the oneDNN
  
  LSTM primitive via REGISTER_XPU_DISPATCH(lstm_mkldnn_stub, ...)


• Extended use_mkldnn() to return true for XPU in inference mode


• Added packed sequence unwrap: when batch_sizes are uniform (common in
  
  batch=1 inference), reshape to regular 3D tensor and use oneDNN path


• Added XPU-specific LSTMCell path with pre-computed input gates as
  
  fallback for training or when oneDNN path is not applicable

The oneDNN primitive processes the entire sequence internally, avoiding

the O(T) kernel launches of the per-step approach.

Depends on intel/torch-xpu-ops#3770 for correct bias handling in the

fused LSTM cell fallback path.