Allow `~/` in path for transform_graph (#15894)

* Allow `~/` in path for transform_graph

This fix tries to address the issue raised in 13211 where
it was not possible to specify `~` (e.g., `~/`, `~user/`, etc)
for the path used in transform_graph. This fix adds
the support of `~` transform_graph on Linux.

This fix fixes 13211.

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

* Reformat transform_graph.cc with clang-format

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