message: Add the comment about FastMem concept.
diff: @@ -151,6 +151,9 @@ class XlaDevice : public LocalDevice {
  const AllocatorAttributes alloc_attrs,
  Tensor* tensor) override LOCKS_EXCLUDED(mu_);
  + // Allocate tensor on fast memory space. This is only applied to the new TPU
  + // hardware which has faster read/write memory. If the hardware doesn't
  + // have such memory space, we fallback to the ordinary memory space.
  Status MakeFastMemTensorFromProto(const TensorProto& tensor_proto,
  const AllocatorAttributes alloc_attrs,
  Tensor* tensor) LOCKS_EXCLUDED(mu_);
  