Modify generate.sh to take a dynamic number of GPUs as input

[aman2304]

Issue

Data generation requires exactly 8 GPUs to be present. This doesn't make the code run properly on machines with less than 8 GPUs (for instance, I am using 4 H100s)

Changes

Changed generate.sh so that the user can specify the number of GPUs as input, with a minimum number of 1. The default is still set to 8 GPUs and the number will change only if the --gpu parameter is specified

[aman2304]

@angelahzyuan could you take a look at this when you have time? Would love to merge it if it looks good to you. Thanks!