Make lazy initialization in plugins more robust

@edward-io is already working on this.

Since Feb 11, 2022.

## 🚀 Feature

We currently set some attributes like sync_batch_norm, num_nodes, devices etc. lazily in the accelerator connector, so the user does not have to provide them at instantiation.

Example:

```python
Trainer(gpus=4, plugins=DDPPlugin(find_unused_parameters=True)) # plugin may require gpus, num_nodes etc.

# AcceleratorConnector does this:
training_type_plugin.num_nodes = ...
```

This is fragile. Some attributes may have to be recomputed based on the order in which others are set.

### Pitch

Provide one single lazy init method that takes all arguments required. The plugin is responsible for making sure dependencies are resolved in one place:

```python
class DDPPlugin(...):

def lazy_init(self, **kwargs):
self.num_nodes = kwargs.get("num_nodes")
self.num_processes = ...

```

With `lazy_init`, the `_configure_launcher` method (#11643) would become obsolete. It can be merged together into `lazy_init`.