Allow obtaining num_nodes from ClusterEnvironment

Nobody has claimed this yet.

## 🚀 Feature
num_nodes must currently be specified manually by the user. However, the number of nodes is generally known in a cluster environment [1] and could be provided by and initialized from [ClusterEnvironment](https://pytorch-lightning.readthedocs.io/en/latest/clouds/cluster.html)

[1] For example $AWS_BATCH_JOB_NUM_NODES in https://docs.aws.amazon.com/batch/latest/userguide/multi-node-parallel-jobs.html

### Pitch

https://github.com/PyTorchLightning/pytorch-lightning/blob/763a9a9495977b23cbd6a57f10253b662fd592a5/pytorch_lightning/plugins/training_type/ddp.py#L62-L75

could be updated to initialize num_nodes from ClusterEnvironment if ClusterEnvironment is provided and implements a `num_nodes` method.

cc @borda @awaelchli @ananthsub

Start in pytorch_lightning/plugins/training_type/ddp.py at the linked num_nodes initialization, then inspect ClusterEnvironment to determine how an optional num_nodes method should be used. The change is complete when num_nodes can be initialized from the cluster environment when that method is available, while manual configuration remains supported otherwise.

Written by the indexing model from the issue text.