Fixed a bug in building valid dataloader

[Taited]

When I ran my training scripts on your newest commit [2a79796], there is a type error in mmgen/apis/train.py :

build_dataloader() got multiple values for keyword argument 'dist'
  File "~/mmgeneration/mmgen/apis/train.py", line 182, in train_model
    val_dataloader = build_dataloader(
  File "~/mmgeneration/tools/train.py", line 217, in main
    train_model(
  File "~/mmgeneration/tools/train.py", line 228, in <module>
    main()

Seems there are redundant dist=distributed and I removed it.

[zengyh1900]

Please help to improve this pull request so that it can be merged @plyfager