minor fix: incorrect assert string message formatting

[bernhardschaefer]

The assert message is split up in 2 strings:

 "SOLVER.IMS_PER_BATCH ({}) must be divisible by the number "
        "of GPUs ({}) used.".format(images_per_batch, num_gpus)

str.format() is only applied to the second string, which produces an incorrect message, where the first placeholder is not substituted and the second is incorrectly substituted.

Example: SOLVER.IMS_PER_BATCH ({}) must be divisible by the number of GPUs (32) used.

[fmassa]

Are you sure that this is necessary? I tried a small example in Python 3 and it worked ok

In [16]: ("a {}"
    ...:  " bb {}".format(0, 10))
    ...:
Out[16]: 'a 0 bb 10'

The reason why we had the string split in two was because of the linter. While I would be ok breaking the linter here, I'd like to understand why my example case works.

[bernhardschaefer]

I realize now that the initial description I gave in the PR is incorrect, sorry for that.
This is not about how str.format() ignores the first string when they are separated, but rather that assert only considers the first string.

Here is a small example:

In[2]: images_per_batch = 11
In[3]: num_gpus = 3
In[4]: assert (
  ...:         images_per_batch % num_gpus == 0
  ...: ), "TEST.IMS_PER_BATCH ({}) must be divisible by the number "
  ...: "of GPUs ({}) used.".format(images_per_batch, num_gpus)
  ...: 
Traceback (most recent call last):
  File "/usr/local/anaconda3/envs/sketch2bot/lib/python3.7/site-packages/IPython/core/interactiveshell.py", line 3291, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-4-38b2e4ea65b1>", line 3, in <module>
    ), "TEST.IMS_PER_BATCH ({}) must be divisible by the number "
AssertionError: TEST.IMS_PER_BATCH ({}) must be divisible by the number 

To show that this is not related to IPython, here is the error I had when running the train script:

AssertionError: SOLVER.IMS_PER_BATCH ({}) must be divisible by the number
Traceback (most recent call last):
  File "tools/train_net.py", line 220, in <module>
    main()
  File "tools/train_net.py", line 213, in main
    model = train(cfg, args.local_rank, args.distributed)
  File "tools/train_net.py", line 62, in train
    start_iter=arguments["iteration"],
  File "/home/ubuntu/maskrcnn-benchmark/maskrcnn_benchmark/data/build.py", line 115, in make_data_loader
    ), "SOLVER.IMS_PER_BATCH ({}) must be divisible by the number "

[bernhardschaefer]

@fmassa I added some clarification and addressed the flake8 issue that you mentioned, feel free to review again.

[fmassa]

Thanks!