Fix giou naming bug

[jdsgomes]

This PR should fix the existing bug caused by the method name being the same as the module name.

before

import torch
from torchvision.ops import generalized_box_iou_loss
generalized_box_iou_loss(torch.zeros((0,4)), torch.zeros((0,4)), reduction="sum")
Traceback (most recent call last):
  File "./IPython/core/interactiveshell.py", line 3457, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-23-efc1ac3887dc>", line 1, in <module>
    generalized_box_iou_loss(torch.zeros((0,4)), torch.zeros((0,4)), reduction="sum")
TypeError: 'module' object is not callable

After PR

import torch
from torchvision.ops import generalized_box_iou_loss
generalized_box_iou_loss(torch.zeros((0,4)), torch.zeros((0,4)), reduction="sum")
tensor(0.)

cc @datumbox

[facebook-github-bot]

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[datumbox]

LGTM, thanks!

[oke-aditya]

Sorry but, I guess this isn't a bug. It's an unreleased feature, so probably label as other?