Improve saving _BatchManager content for cache

[gabrielmbmb]

Right now, we're dumping the whole _BatchManager in a JSON file. This is fine for pipelines that doesn't generate too much data, but doesn't work well for pipeline generating too much data (embeddings for example).

We need to dump the data of each _BatchManagerStep in a different file (or files), so when loading the _BatchManager back we don't have issues.