Deprecate `save` from the Logger Base API

Nobody has claimed this yet.

## Proposed refactor

Deprecate `save` from `LightningLoggerBase`:
https://github.com/PyTorchLightning/pytorch-lightning/blob/a8ee5cacb7b31064c8b73372ca944d43f7caadc6/pytorch_lightning/loggers/base.py#L173-L175

### Motivation

After https://github.com/PyTorchLightning/pytorch-lightning/issues/9145 `save` on the `LightningLoggerBase` does nothing, and only 2 loggers have implementations for it: `CSVLogger` and `TensorboardLogger`.

After https://github.com/PyTorchLightning/pytorch-lightning/issues/8991 we now consider flushing an internal implementation detail of the loggers, so it follows that `save` should also be an internal implementation detail of the loggers.

Currently the Trainer calls `log_metrics` and `save` together here:
https://github.com/PyTorchLightning/pytorch-lightning/blob/6bc0e1da6329c42ec3b7c6fbf91eb059b9124207/pytorch_lightning/trainer/connectors/logger_connector/logger_connector.py#L131-L132

Instead, the Trainer can just call `log_metrics` and the actual saving can be delegated completely to the logger, including buffering & flush frequency.

### Pitch
in v1.6:
* Deprecate `save` from `LightningLoggerBase`
* For the following three `save` calls, only call if `save` is overridden in the Logger:
https://github.com/PyTorchLightning/pytorch-lightning/blob/6bc0e1da6329c42ec3b7c6fbf91eb059b9124207/pytorch_lightning/trainer/connectors/logger_connector/logger_connector.py#L132
https://github.com/PyTorchLightning/pytorch-lightning/blob/6bc0e1da6329c42ec3b7c6fbf91eb059b9124207/pytorch_lightning/trainer/trainer.py#L1241
https://github.com/PyTorchLightning/pytorch-lightning/blob/6bc0e1da6329c42ec3b7c6fbf91eb059b9124207/pytorch_lightning/loops/epoch/training_epoch_loop.py#L508
(Note: the final `save` call above will be removed in v1.7 following the removal of `flush_logs_every_n_steps`)

in v1.8:
* For `CSVLogger` and `TensorboardLogger` their `log_metrics` and `log_graph` implementations should call `save`
* Remove `save` from `LightningLoggerBase`

### Additional Context
This was discussed in https://github.com/PyTorchLightning/pytorch-lightning/discussions/9004

cc @justusschock @awaelchli @akihironitta @rohitgr7 @edward-io @borda @ananthsub @kamil-kaczmarek @Raalsky @Blaizzy @tchaton

Start with pytorch_lightning/loggers/base.py and the three save call sites in logger_connector.py, trainer.py, and training_epoch_loop.py. Compare the Logger implementations for CSVLogger and TensorboardLogger, then review the linked discussions and issues for the intended deprecation sequence. Done means the v1.6 save-call behavior is updated without breaking logger tests, with the later removal work clearly separated.

Written by the indexing model from the issue text.