Why and when patch_builtin_len is needed?

[hujh1994]

detectron2/detectron2/export/torchscript_patch.py

Line 316 in 0e29b7a

def patch_builtin_len(modules=()):

I try to understand this function. Not sure my understanding is right which is when the patch_builtin_len is called in a with, module.len is changed to module.__len__ in the field managed by the with, where module belongs to MODULES in

detectron2/detectron2/export/torchscript_patch.py

Line 336 in 0e29b7a

ctxs = [stack.enter_context(mock.patch(mod + ".len")) for mod in MODULES]

, like detectron2.modeling.roi_heads.fast_rcnn.
My questions are:

1. Is my understanding right?
2. Does detectron2.modeling.roi_heads.fast_rcnn have __len__? I tried to import it in Ipython and found that it didn't have __len__.
3. Why patch_builtin_len is needed? For example, in

   detectron2/detectron2/export/flatten.py

   Line 280 in cbbc1ce

   with torch.no_grad(), patch_builtin_len():

   .