tfdbg: allow writing tensor values to filesystem as numpy files

Command example:
tfdbg> pt Softmax:0 -w /tmp/softmax.npy
tfdbg> eval 'np.sum(`Softmax:0`, axis=1)' -w /tmp/softmax_sum.npy

The saved file can later be loaded in Python:
>> import numpy as np
>> np.load('/tmp/softmax.npy')
>> np.load('/tmp/softmax_sum.npy')

RELNOTES: For `pt` and `eval` commands, allow writing tensor values to filesystem as numpy files.
PiperOrigin-RevId: 179817453