Fix segmentation fault with tf.stack an keras's Input in TF2.0
Fix segmentation fault with tf.stack an keras's Input in TF2.0

This fix tries to address the issue raised in 26879 where
tf.stack encounters segmantation fault with keras's Input in TF2.0:
```
import tensorflow as tf
from tensorflow.keras.layers import Input

print(tf.__version__)

input_ = Input((128, 128, 1), dtype='float32')
print(input_)
output = tf.stack(input_, axis=1)
```

The issue is that in eager wrap, `PySequence_Fast_GET_ITEM`
tries to access an object not through `PySequence_Fast`.

This fix adds the `PySequence_Fast` and checks the return value
to make sure it is not nullptr.

This fix fixes 26879.

Signed-off-by: Yong Tang <765086fe2e0c1f980161f127fec596800f327f62@outlook.com>

annarev/tensorflow,jhseu/tensorflow,yongtang/tensorflow,aam-at/tensorflow,DavidNorman/tensorflow,chemelnucfin/tensorflow,freedomtan/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,karllessard/tensorflow,adit-chandra/tensorflow,cxxgtxy/tensorflow,aldian/tensorflow,DavidNorman/tensorflow,kevin-coder/tensorflow-fork,sarvex/tensorflow,tensorflow/tensorflow-pywrap_saved_model,aam-at/tensorflow,annarev/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,annarev/tensorflow,aldian/tensorflow,renyi533/tensorflow,arborh/tensorflow,annarev/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,paolodedios/tensorflow,chemelnucfin/tensorflow,frreiss/tensorflow-fred,adit-chandra/tensorflow,gunan/tensorflow,arborh/tensorflow,davidzchen/tensorflow,ghchinoy/tensorflow,frreiss/tensorflow-fred,gautam1858/tensorflow,DavidNorman/tensorflow,petewarden/tensorflow,tensorflow/tensorflow-pywrap_saved_model,aam-at/tensorflow,DavidNorman/tensorflow,karllessard/tensorflow,frreiss/tensorflow-fred,ppwwyyxx/tensorflow,chemelnucfin/tensorflow,tensorflow/tensorflow,ppwwyyxx/tensorflow,kevin-coder/tensorflow-fork,adit-chandra/tensorflow,Intel-tensorflow/tensorflow,xzturn/tensorflow,jhseu/tensorflow,davidzchen/tensorflow,karllessard/tensorflow,aam-at/tensorflow,renyi533/tensorflow,arborh/tensorflow,renyi533/tensorflow,ppwwyyxx/tensorflow,renyi533/tensorflow,chemelnucfin/tensorflow,tensorflow/tensorflow-pywrap_saved_model,Intel-tensorflow/tensorflow,xzturn/tensorflow,kevin-coder/tensorflow-fork,Intel-tensorflow/tensorflow,Intel-Corporation/tensorflow,ppwwyyxx/tensorflow,tensorflow/tensorflow-pywrap_saved_model,ppwwyyxx/tensorflow,karllessard/tensorflow,Intel-tensorflow/tensorflow,yongtang/tensorflow,yongtang/tensorflow,frreiss/tensorflow-fred,kevin-coder/tensorflow-fork,alsrgv/tensorflow,ghchinoy/tensorflow,tensorflow/tensorflow,jhseu/tensorflow,xzturn/tensorflow,annarev/tensorflow,davidzchen/tensorflow,ghchinoy/tensorflow,arborh/tensorflow,Intel-Corporation/tensorflow,freedomtan/tensorflow,davidzchen/tensorflow,DavidNorman/tensorflow,freedomtan/tensorflow,xzturn/tensorflow,freedomtan/tensorflow,xzturn/tensorflow,tensorflow/tensorflow,gunan/tensorflow,Intel-tensorflow/tensorflow,sarvex/tensorflow,kevin-coder/tensorflow-fork,DavidNorman/tensorflow,Intel-Corporation/tensorflow,gunan/tensorflow,yongtang/tensorflow,alsrgv/tensorflow,gunan/tensorflow,Intel-Corporation/tensorflow,paolodedios/tensorflow,yongtang/tensorflow,paolodedios/tensorflow,petewarden/tensorflow,alsrgv/tensorflow,cxxgtxy/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,cxxgtxy/tensorflow,annarev/tensorflow,frreiss/tensorflow-fred,ghchinoy/tensorflow,freedomtan/tensorflow,renyi533/tensorflow,paolodedios/tensorflow,jhseu/tensorflow,ghchinoy/tensorflow,chemelnucfin/tensorflow,sarvex/tensorflow,cxxgtxy/tensorflow,tensorflow/tensorflow-pywrap_saved_model,paolodedios/tensorflow,alsrgv/tensorflow,gunan/tensorflow,tensorflow/tensorflow,frreiss/tensorflow-fred,kevin-coder/tensorflow-fork,aam-at/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,davidzchen/tensorflow,xzturn/tensorflow,ghchinoy/tensorflow,DavidNorman/tensorflow,kevin-coder/tensorflow-fork,kevin-coder/tensorflow-fork,arborh/tensorflow,karllessard/tensorflow,davidzchen/tensorflow,frreiss/tensorflow-fred,alsrgv/tensorflow,adit-chandra/tensorflow,tensorflow/tensorflow-pywrap_saved_model,tensorflow/tensorflow-experimental_link_static_libraries_once,gautam1858/tensorflow,yongtang/tensorflow,jhseu/tensorflow,karllessard/tensorflow,adit-chandra/tensorflow,arborh/tensorflow,arborh/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,ghchinoy/tensorflow,DavidNorman/tensorflow,gunan/tensorflow,yongtang/tensorflow,alsrgv/tensorflow,aam-at/tensorflow,tensorflow/tensorflow,frreiss/tensorflow-fred,tensorflow/tensorflow-pywrap_saved_model,annarev/tensorflow,aam-at/tensorflow,freedomtan/tensorflow,ppwwyyxx/tensorflow,arborh/tensorflow,tensorflow/tensorflow-pywrap_saved_model,tensorflow/tensorflow-pywrap_saved_model,ppwwyyxx/tensorflow,ghchinoy/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,Intel-Corporation/tensorflow,annarev/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,yongtang/tensorflow,gunan/tensorflow,tensorflow/tensorflow-pywrap_saved_model,gautam1858/tensorflow,arborh/tensorflow,renyi533/tensorflow,karllessard/tensorflow,aldian/tensorflow,adit-chandra/tensorflow,adit-chandra/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,xzturn/tensorflow,petewarden/tensorflow,paolodedios/tensorflow,jhseu/tensorflow,freedomtan/tensorflow,gautam1858/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,annarev/tensorflow,petewarden/tensorflow,davidzchen/tensorflow,gunan/tensorflow,davidzchen/tensorflow,chemelnucfin/tensorflow,arborh/tensorflow,sarvex/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,aldian/tensorflow,renyi533/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,freedomtan/tensorflow,jhseu/tensorflow,gautam1858/tensorflow,cx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