Add int64 support for BroadcastArgs and BroadcastGradientArgs (#15255)

* Add int64 support for BroadcastArgs and BroadcastGradientArgs

In `array_ops.cc`, both int32 and int64 are expected to be supported
for `BroadcastArgs` and `BroadcastGradientArgs`. However, this was
not the case as only int32 kernel are registered even though `T`
is part of the `TypeConstraint`.

This fix adds the int64 kernel support for `BroadcastArgs` and
`BroadcastGradientArgs`, and adds related test cases.

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

* Add test cases for int64 support of BroadcastArgs and BroadcastGradientArgs

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

* Sanitize python and clang-format.

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