Added script to download train data

[ElisonSherton]

Created a python script to perform the following task

• Download zip datasets from insightface repository for ms1mv2, ms1mv3 & casia_webface.
• Unzip the datasets and read the train.rec and train.idx files respectively.
• Create dataset in the form mentioned in the README.md file
• Deleted the downloaded zip file and the unnecessary extracted files

I have ran the script myself with all three datasets and it is working fine. I have an Ubuntu:20.04.6 LTS OS and a Dell Latitude series machine on which I performed this test. I benefitted from this repository and wanted to contribute this script so that it is easy for newcomers to set up the training data as well on their machine.