Bug Fix : Dataset Size

[kshitijrajsharma]

What does this PR do ?

• Fixes the bug that introduced after we move away from the raw folder structure to .tar method
• Fixes Dataset size is not showing #245

Consideration :

I have decided to use chips_length to make it meaningful instead of dataset_size in the database as we have dataset model already and it might create confusion

• Rebased on top of Feature : Seperation of API and workers #250

How to test ?

• Run training and it should have a new field called chips_length after you call the api /training/{training_id}