💾 viable/strict/1780997283: Add oneDNN LSTM primitive support for XPU inference (#185531)

Summary
Use dnnl::lstm_forward primitive on XPU for LSTM inference, replacing the per-timestep fused cell approach. The oneDNN primitive processes the entire sequence internally, eliminating O(T) kernel launches.
Changes

New file aten/src/ATen/native/mkldnn/xpu/RNN.cpp: Implements lstm_onednn_xpu using dnnl::lstm_forward, registered via...