Refactor TensorArray to avoid copies and memory allocations when executing eagerly.

With this change, writes to TensorArrays when eager execution is enabled take O(1) time instead of O(n). Additionally, whereas writing to a TensorArray when constructing a graph results in allocating a new Python TensorArray object, writing to a TensorArray with eager enabled no longer performs that allocation (graph construction uses these allocations to ensure correctness of control flow and gradients, but this isn't necessary when executing eagerly). Finally, this change also removes the artificial write-once semantics of TensorArrays when executing eagerly.

PiperOrigin-RevId: 195739572