How do we understand '_C' in adet?

Sometimes you see snippets of code like this: DETR

from adet import _C
...
class _MSDeformAttnFunction(torch.autograd.Function):
    @staticmethod
    def forward(ctx, value, value_spatial_shapes, value_level_start_index, sampling_locations, attention_weights, im2col_step):

        output = _C.ms_deform_attn_forward(
            value, value_spatial_shapes, value_level_start_index, sampling_locations, attention_weights, ctx.im2col_step)

So what exactly does _C stand for? Where is the source code for this part?

mlpc-ucsd / TESTR

How do we understand '_C' in adet? #16