Add py.typed for PEP 561 compliance

[byarbrough]

What does this PR do?

Fixes #5325

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline?
• Did you read our philosophy doc (important for complex PRs)?
• Was this discussed/approved via a Github issue or the forum? Please add a link to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings. not required
• Did you write any new necessary tests? none required

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

@patrickvonplaten

[byarbrough]

I confirmed this works with pip install -e .

mypy

Running against file presented in issue

Success: no issues found in 1 source file

pyright

Running pyright --verifytypes diffusers

Module name: "diffusers"
Package directory: "/tmp//diffusers/src/diffusers"
Module directory: "/tmp/diffusers/src/diffusers"
Path of py.typed file: "/tmp/diffusers/src/diffusers/py.typed"

Public modules: 302

Symbols exported by "diffusers": 9018
  With known type: 1307
  With ambiguous type: 505
  With unknown type: 7206
  Functions without docstring: 1403
  Functions without default param: 318
  Classes without docstring: 378

Other symbols referenced but not exported by "diffusers": 4390
  With known type: 1602
  With ambiguous type: 138
  With unknown type: 2650

Type completeness score: 14.5%

[patrickvonplaten]

Thanks!

[byarbrough]

I'm hoping this isn't the case here, but found today that this didn't work well with huggingface/transformers#18485