It's sometimes difficult to initialize pipeline components in code

The workflow for setting up a pipeline component in code sometimes feels a bit rough. This came up while I was investigating #6958.

Let's say we have some pipeline component that assumes its `.initialize()` method will be called before it's in a valid state, as the transformer does --- but the component doesn't necessarily need to be _trained_, as such, before it's in a functional state. We have the following:

```python

import spacy

nlp = spacy.blank("en")
transformer = nlp.add_pipe("transformer")
```

So now we need to call `transformer.initialize()`. How to do that?

* Maybe I should use `nlp.initialize()`? That does work --- but if I were adding the component in addition to _other_ components, I'll have problems, as I'll wipe their weights.
* Maybe I should use `nlp.resume_training()`? It seemed like that ought to work, even though it's not the most obvious. It doesn't though, because it doesn't call `.initialize()` on the components, as it can't know what weights that would reset.
* Okay so maybe I should call `transformer.initialize(get_examples=lambda: [], nlp=nlp)`. However, this runs into an error in `validate_get_examples`, which complains the list is empty. The component does support an empty list though.
* `transformer.initialize(nlp=nlp)`? This doesn't work, even though the docstring refers to it as an "optional `get_examples` callback".
* Okay so what I need to do is construct at least one `Example` object, so that I can return it in `get_examples`. Kind of a hassle.
* Alternatively I could be sneaky and do `transformer.model.initialize()`. This happens to work, but if the component requires other initialization it won't in this instance, so it's not a generalizable solution.

A quick improvement is to add an argument to `validate_get_examples` indicating whether the component can work with no examples. I'm not sure how to help components that do need some data though.

Maybe some components should check whether they're initialized, and do that on first usage if necessary? It does feel dirty, though.