Fix stream race in dispatch/combine with deterministic algorithms

[ccs668899]

The cause of #589 so far as I can tell is as follows in deep_ep.cpp:

// The comm stream waits for everything on the compute stream.
stream_wait(comm_stream, compute_stream)

// The output tensor is allocated, but in deterministic mode a NaN-fill is enqueued on the compute stream.
torch::empty(recv_x)

// The communication kernel writes recv_x on to the comm stream.
intranode::combine(..., recv_x, ..., comm_stream)

Now this could be fixed by putting the stream wait until after the empty allocation, but we have the comment "NOTES: do not allocate tensors upfront!". I don't want to mess up the stream ownership so I just add a second sync stream wait right before the intranode combine:

// The output tensor is allocated, but in deterministic mode a NaN-fill is enqueued on the compute stream.
torch::empty(recv_x)

// Ensure comm_stream observes all compute_stream work above this point.
if (!allocate_on_comm_stream)
    stream_wait(comm_stream, compute_stream);

// The communication kernel writes recv_x on to the comm stream.
intranode::combine(..., recv_x, ..., comm_stream)

The tests should deterministically catch the bug once the fix is in. I don't have a multi-GPU setup on hand to validate myself, any recommendations?