Add minimum version check in examples
# What does this PR do?

This PR adds a minimum version check to all examples, in order to avoid the waves of issues created each time they use a functionality that was just released into `Trainer`. The script will immediately error if the version of Transformers does not match the required minimum version. At each release, a script will set that to the version released automatically (work in progress for a second PR with other release utils) so that the examples associated with one tag will require the minimum version of that tag.

The user can still remove that line to avoid the error (at their own risks). The error points out to:
- the instruction for a source install
- the examples README that now lists all examples folders with the various version tags.
