TorchModel typing for fname argument in save is str, but is used as pathlib.PosixPath

[yurakuratov]

fname argument of TorchModel.save() is expected to be str, but is actually used as pathlib.PosixPath https://github.com/deepmipt/DeepPavlov/blob/master/deeppavlov/core/models/torch_model.py#L172

Support, please, both types of arguments or fix typing.