Auto dataloader - ValueError: batch_size should be a positive integer value, but got batch_size=0

🐛 Bug description

The following code will brake if batch_size is smaller than world size :

import torch
import ignite.distributed as idist

def foo(i):
    data = torch.arange(100).reshape(25, 4)
    data_loader = idist.auto_dataloader(data, batch_size=6, num_workers=12)


if __name__ == "__main__":
    idist.spawn("gloo", foo, args=(), nproc_per_node=8)    

ValueError: batch_size should be a positive integer value, but got batch_size=0

A fix can be:

• keep batch size as provided if smaller than world size
• same for num_workers

Environment

• PyTorch Version (e.g., 1.4): 1.5.0
• Ignite Version (e.g., 0.3.0): master
• OS (e.g., Linux): linux
• How you installed Ignite (conda, pip, source):
• Python version: 3.7
• Any other relevant information:

@InCogNiTo124 would you like to fix this as you recently played around auto_* ?

[InCogNiTo124]

@vfdev-5 alright, I’ll let you know if I can’t fix it by then 😃

[vfdev-5]

@InCogNiTo124 yes, you are correct about that: 6 // 8 = 0

A fix cant be:

@vfdev-5 I’m not sure if this is a typo or not

And yes, it is a typo. I fixed it.