Added examples for fill_mode (#8398)
Added examples for fill_mode (#8398)

* Added examples for fill_mode

It wasn't 100% clear to me what "reflect" and "wrap" modes do exactly so I looked in scipy source for examples. The `fill_mode` is implemented using `NI_ExtendLine()` from [ni_support.c](https://github.com/scipy/scipy/blob/master/scipy/ndimage/src/ni_support.c).

* Updated fill_mode examples

Added code delimiters and `cval=k` to `fill_mode` examples.

keras-team/keras,keras-team/keras