In PjRtStreamExecutorBuffer::Delete, fix some bugs causing memory corruption.
In PjRtStreamExecutorBuffer::Delete, fix some bugs causing memory corruption.

The bug is that we don't wait for events using the buffer on the Compute stream, and also the definition events. This can cause a race condition where the buffer is deleted before it is read/written, which can lead to memory corruptions.

For events on the compute stream, the fix is to schedule the deallocation on the compute stream.
For the definition events, the fix is to wait for them on a borrowed stream if they are not defined on the compute stream.

This change probably wouldn't be needed if all writes are scheduled on the compute stream, but that is not the case today. We have a lot of usages of the h2d and d2d stream.
