Add empty __init__.pys where missing

[wchargin]

Summary:
Bazel generates these automatically, but having them in the source tree
is convenient for REPLs and tools like pydoc.

Generated with:

find tensorboard -type f -name '*.py' |
    xargs -n 1 -d '\n' dirname |
    sort -u |
    xargs -n 1 -d '\n' sh -c 'cp -n tensorboard/util/__init__.py "$1/"' cmd

Test Plan:
Running pydoc tensorboard.data.provider after this commit successfully
shows useful docs; before this commit, it fails with, “no Python
documentation found for 'tensorboard.data.provider'”.

wchargin-branch: empty-init-py

[stephanwlee]

All those licenses on empty init's :)

[wchargin]

Yeah… At least I think they don’t make it into the final Pip package,
because we never depend on the __init__.pys at the BUILD level. So
we’re not shipping kilobytes of redundant licenses to end users, just
ourselves. :-)

[nfelt]

FWIW Bazel's behavior of auto-creating the __init__.py is in theory going to be deprecated and removed eventually: bazelbuild/bazel#7386

[wchargin]

Good to know; thanks. Perhaps as that time draws near we can write a
test that in the extracted //tensorboard/pip_package, every directory
with a *.py file also has an __init__.py file.