Remove redundant quant/dequant in GenrealizedRCNN

Summary: Removed quant/dequant between backbone and proposal generator, and roi_box_conv and the following avg_pool

Reviewed By: wat3rBro

Differential Revision: D29383036

fbshipit-source-id: ef07b3d1997b1fc7f92bcd9201523e9071510a8b

Author: jerryzh168

detectron2/export/c10.py

@@ -299,8 +299,12 @@ def forward(self, x, box_lists):
                 c2_roi_align = torch.ops._caffe2.RoIAlign
                 aligned = self.level_poolers[0].aligned
 
+            x0 = x[0]
+            if x0.is_quantized:
+                x0 = x0.dequantize()
+
             out = c2_roi_align(
-                x[0],
+                x0,
                 pooler_fmt_boxes,
                 order="NCHW",
                 spatial_scale=float(self.level_poolers[0].spatial_scale),
@@ -337,6 +341,9 @@ def forward(self, x, box_lists):
                 c2_roi_align = torch.ops._caffe2.RoIAlign
                 aligned = bool(pooler.aligned)
 
+            if x_level.is_quantized:
+                x_level = x_level.dequantize()
+
             roi_feat_fpn = c2_roi_align(
                 x_level,
                 roi_fpn,