Tensorflow/GCS: Check whether we are running under GCE before trying to obtain...

Tensorflow/GCS: Check whether we are running under GCE before trying to obtain auth token from GCE. Before this change, if a process is not running under GCE, the token request to http://metadata server would time out after 3+ minutes of retry. After this change, the check is bypassed, and we return an empty token to the caller. At that point, the caller's request to read/write a file in GCS would either succeed or fail depending on whether the bucket is publicly accessible.

PiperOrigin-RevId: 189900977