Implement embeddings calculation "on the fly"

[SebieF]

Currently, embeddings must be pre-calculated before the training process starts. For some users, it might be beneficial to calculate the embeddings on the fly, especially if they only require low GPU capacities.

TODOs:

• Identify a relevant use-case (@sacdallago ?)
• Add option to calculate embeddings on the fly in config file
• Modify dataset class accordingly
• Modify trainer pipeline accordingly