[PJRT:GPU] Don't use XLA transfer manager APIs to perform H2D copies.
[PJRT:GPU] Don't use XLA transfer manager APIs to perform H2D copies.

This fixes a regression introduced by https://github.com/openxla/xla/commit/36fea6d51809cee927830064e60383d0a1aef041, which had the effect of meaning we were bouncing buffers via pinned memory twice, and performing synchronous chunking. This was a large net slowdown.

However, really our intent here was just to call to perform a H2D transfer and nothing else, so we can just call the StreamExecutor APIs directly.
