Fix FA3 segfault with custom CUDA streams in ABI stable build (#1957)

The ABI stable implementation incorrectly used getCurrentStream().id()
which returns a StreamId (int64_t) instead of the actual cudaStream_t
pointer. Casting an integer ID to a stream pointer caused segmentation
faults when using custom CUDA streams.

Fixed by using the proper AOTI C API function aoti_torch_get_current_cuda_stream()
which returns the actual CUDA stream pointer.