add in silico perturbation module

Add gene name : Ensembl ID dictionary

Fix bug with metadata when processing multiple .loom files (#3)

Update token_dictionary.pkl

Delete geneformer/token_dictionary.pkl

Add data collator for gene classification and example for gene classification

Add data collator for cell classification and example for cell classification

Add Geneformer trainer and pretraining example

Add Geneformer tokenizer and updated model card