[tf2xla] Introduce XlaTensorInfo

XlaTensorInfo is side-band data for Tensors. It can be used to store
information about Tensors that is not possible to store in the Tensor
itself.  The XlaTensorInfos are managed by XlaTensorInfoManager, which
is an Allocator, which allows it to release the TensorInfos when the
underlying Tensor is released.  Looking up an XlaTensorInfo for a
Tensor requires a hash table lookup. This implementation keeps this
off the fast path and only looks the tensorinfos up when they are
required.

PiperOrigin-RevId: 189319553