Introduce consistent handling of warm_up for components

[mathislucka]

This issue was brought up by @davidsbatista during the review of

We defer time-consuming or resource expensive operations when initializing components so that loading of a pipeline to validate it is a fast and lightweight operation.

Instead of performing expensive operations in a component's __init__-method, we advise to move them to a warm_up-method. The warm_up-method should only be called when the intention is to actually run a component.

We also advise that components should only perform an expensive warm_up once.
For example, our embedders and rankers only load and initialize their model when it wasn't loaded and initialized before.

In the run-method of our Pipeline, we call warm_up on each component, to make sure that we don't run any components that haven't been warmed up.

However, if a component doesn't respect our guideline of checking if it has been warmed up before, this might cause slow operations to run on every invocation of the pipeline's run-method.

We should agree on an approach to make sure that this doesn't happen.

[sjrl]

@julian-risch I believe your issue from here is also related. Perhaps we can merge the two?

[julian-risch]

I closed the other issue as a duplicate now. Here is context from the other issue.

Currently, the pipeline calls warmup on all its components that implement warm_up when the pipeline is run. Every time.
While we say in PipelineBase:

It's the node's responsibility to make sure this method can be called at every Pipeline.run() without re-initializing everything.

There is also a ToDo in the pipeline implementation stating:

    # TODO: Remove this warmup once we can check reliably whether a component has been warmed up or not
    # As of now it's here to make sure we don't have failing tests that assume warm_up() is called in run()

We should make this more consistent, for example by introducing an is_warmed_up() to components that implement warm_up(). Could be hidden from the user and automatically set to return True after warm_up() was called for the first time.