Cannot seek on write only tf.gfile.GFile

System information

• Have I written custom code: Yes
• OS Platform and Distribution: Linux Ubuntu 18.04
• TensorFlow installed from: binary
• TensorFlow version: v1.14.0-rc1-22-gaf24dc9 1.14.0
• Python version: 3.6

Describe the current behavior
Calling seek() on a tf.gfile.GFile opened in write only mode raises tensorflow.python.framework.errors_impl.PermissionDeniedError.

Describe the expected behavior
GFile should support the Python IO semantics that supports seeking on a write only file.

More generally it would be preferable if GFile followed the API of Python's io.IOBase.

Code to reproduce the issue

import tensorflow as tf

with tf.io.gfile.GFile('test.txt', 'w') as f:
    f.seek(0)

Other info / logs

Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "/VENV/lib/python3.6/site-packages/tensorflow/python/util/deprecation.py", line 507, in new_func
    return func(*args, **kwargs)
  File "/VENV/lib/python3.6/site-packages/tensorflow/python/lib/io/file_io.py", line 146, in seek
    self._preread_check()
  File "/VENV/lib/python3.6/site-packages/tensorflow/python/lib/io/file_io.py", line 82, in _preread_check
    "File isn't open for reading")
tensorflow.python.framework.errors_impl.PermissionDeniedError: File isn't open for reading