feat(pipeline): add progress bar and check on sampler

[mspronesti]

Hello @kjsman , another PR to fix a small issue related to a missing check on the sampler passed to the pipeline and to add a progress bar, which might be of help when using this codebase, especially on a CPU.

CHANEGELOG:

• add tqdm progress bar to track generation status
• check sampler value is legit otherwise throw ValueError. Without this check, the pipeline crashes when attempting to call a method of a sampler object (sampler is still a string in this scenario)
• add tqdm to requirements.txt