Update the _test_requirements to use the dev version of Accelerate
# What does this PR do?

Since there is rapid development of the Accelerate package, this includes the git version as a requirement for the examples in `_test_requirements.txt` for the examples, and also updates the CI to use that for installation only.

## 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.

@sgugger @ydshieh 

_The documentation is not available anymore as the PR was closed or merged._