Ignite: Fix mypy issue with pytorch nightly

[vfdev-5]

🐛 Bug description

Currently, there is an issue with running mypy on ignite when using pytorch-nightly :

 ignite/distributed/comp_models/native.py:334: error: Incompatible types in
assignment (expression has type "ReduceOp", variable has type "str") 
[assignment]
                op = self._reduce_op_map[op]
                     ^
Found 1 error in 1 file (checked 84 source files)

https://github.com/pytorch/ignite/runs/1373059337#step:5:17

@gruebel could you please take a look if we can fix it ? Thanks

[gruebel]

Sure, I can take a look today

[vfdev-5]

Thanks @gruebel ! And couldy you please also remove namedtuple import (https://github.com/pytorch/ignite/pull/1433#discussion_r516991700). I merged too fast that PR :)

[gruebel]

yeah, good point. Already forgot about it 🙈