tfdbg: stepper: use dumped intermediate tensors during cont() calls

Each cont() call will use TFDBG's own RunOptions.debug_options to generate intermediate tensor dumps and load the dumps and cached the relevant DebugTensorDatum objects. If future cont() calls require the intermediate tensor values, they will be obtained from the cached DebugTensorDatum objects, saving unnecessary recomputation.

The use of such cached intermediate tensor dumps can be disabled in individual cont() calls by using "use_dumped_intermediates=False".
Change: 146572672