Fix: Slide ids turned into floats in split csv when names consist of only number

[ff98li]

Summary of the Issue

• Slide IDs consisting solely of numerical characters are inadvertently converted to floats in the split CSV files
  • The unequal lengths of train, val, and test splits introduce NaN values when these splits are concatenated into a dataframe by save_splits().
  • Pandas automatically converts columns with all-numeric names and NaN values to floats due to the lack of NaN rep in integer columns in Pandas.
• When loading via the following line, ValueError as shown in the screenshot will occur https://github.com/mahmoodlab/CLAM/blob/3f875f77465b410d260f2afcfaea608a9d6ddbca/datasets/dataset_generic.py#L247

Proposed fix

• Cast slide IDs to strings before being saved to CSV in save_splits to prevent unintended type conversion.
  • Result:
• Continue to read the dataset CSV with dtype=object in Generic_WSI_Classification_Dataset.
  • In get_split_from_df(), cast the dtype of the corresponding split column to match that of self.slide_data['slide_id'].
  • This fix is pertaining https://github.com/mahmoodlab/CLAM/pull/90
  • Result:

This happened when I was working with my own task's dataset csv. I can provide the csv file to reproduce this bug if needs be.