Add annotations support for tf.estimator.Estimator (#12276)

* Add annotations support for tf.estimator.Estimator
This fix adds annotations support for tf.estimator.Estimator
so that the following works in python 3:
```
import tensorflow as tf

def model_fn(features: dict, labels: tf.Tensor, mode: str):
    pass

estimator = tf.estimator.Estimator(model_fn)
```

This fix fixes 12249.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Update tf_inspect with getfullargspec

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>