📰 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()...