[OpenCL] Stats tracking (#11523)

* Adds stat tracking to the SYCL allocator

The SYCLAllocator will now find the max allocation size on construction,
and keep track of the allocation stats, as given in AllocationStats.

* [OpenCL] Adds buffer size tracking to SYCL allocator (#114)

The SYCL buffers underlying tensors already keep track of their sizes,
so we can easily provide this tracking information for debugging
purposes.