Add support for hierarchical dict
🚀 Feature
Motivation
Since v0.7.0, LightningModule accepts dict hparams, however, still TensorBoardLogger raises an error with hierarchical dict. Considering the compatibility of the other package, especially Hydra #807, hierarchical dict should be accepted by any loggers.
Pitch

Flatten hierarchical dict before hparam logging

Alternatives
The function _convert_params in loggers/base.py will be changed like:
def _convert_params(self, params: Union[Dict[str, Any], Namespace]) -> Dict[str, Any]:
    # in case converting from namespace
    if isinstance(params, Namespace):
        params = vars(params)

    # added part
    params = flatten_dict(params)  # convert {'a': {'b': 'c'}} -> {'a/b': 'c'}

    if params is None:
        params = {}

    return params