Error when trying to train FasterRCNN with custom backbone on GRAYSCALE images
I am following instructions from https://pytorch.org/tutorials/intermediate/torchvision_tutorial.html#putting-everything-together tutorial in order to create object detector for 1 class on GRAYSCALE images.
Here is my code (note that I am using DenseNet as a BACKBONE - pretrained model by me on my own dataset):
device = torch.device('cuda') if torch.cuda.is_available() else torch.device('cpu')

num_classes = 2 # 1 class + background

model = torch.load(os.path.join(patch_classifier_model_dir, &quot;densenet121.pt&quot;))

backbone = model.features

backbone.out_channels = 1024

anchor_generator = AnchorGenerator(sizes=((32, 64, 128, 256, 512),),

                                   aspect_ratios=((0.5, 1.0, 2.0),))

roi_pooler = torchvision.ops.MultiScaleRoIAlign(featmap_names=[0],

                                                output_size=7,

                                                sampling_ratio=2)

# put the pieces together inside a FasterRCNN model

model = FasterRCNN(backbone,

                   num_classes=2,

                   rpn_anchor_generator=anchor_generator,

                   box_roi_pool=roi_pooler)

# move model to the right device

model.to(device)

optimizer = torch.optim.SGD(model.parameters(), lr=0.005,

                            momentum=0.9, weight_decay=0.0005)

# and a learning rate scheduler which decreases the learning rate by

# 10x every 3 epochs

lr_scheduler = torch.optim.lr_scheduler.StepLR(optimizer,

                                               step_size=3,

                                               gamma=0.1)

This is the error that I am running into:
RuntimeError: Given groups=1, weight of size [64, 1, 7, 7], expected input[2, 3, 1344, 800] to have 1 channels, but got 3 channels instead
Based on FasterRCNN architecture, I assume problem is in the transform component because it tries to normalize images that are initially grayscale, and not RGB:
FasterRCNN(
  (transform): GeneralizedRCNNTransform(
      Normalize(mean=[0.485, 0.456, 0.406], std=[0.229, 0.224, 0.225])
      Resize(min_size=(800,), max_size=1333, mode='bilinear')
  )
  (backbone): Sequential(
    (conv0): Conv2d(1, 64, kernel_size=(7, 7), stride=(2, 2), padding=(3, 3), bias=False)
    (norm0): BatchNorm2d(64, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
    (relu0): ReLU(inplace=True)
    (pool0): MaxPool2d(kernel_size=3, stride=2, padding=1, dilation=1, ceil_mode=False)
    (denseblock1): _DenseBlock(
      (denselayer1): _DenseLayer(
        (norm1): BatchNorm2d(64, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
        (relu1): ReLU(inplace=True)
        (conv1): Conv2d(64, 128, kernel_size=(1, 1), stride=(1, 1), bias=False)
        (norm2): BatchNorm2d(128, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
        (relu2): ReLU(inplace=True)
        (conv2): Conv2d(128, 32, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1), bias=False)
      )
      
      ...............
        
    (norm5): BatchNorm2d(1024, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
  )
  (rpn): RegionProposalNetwork(
    (anchor_generator): AnchorGenerator()
    (head): RPNHead(
      (conv): Conv2d(1024, 1024, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
      (cls_logits): Conv2d(1024, 15, kernel_size=(1, 1), stride=(1, 1))
      (bbox_pred): Conv2d(1024, 60, kernel_size=(1, 1), stride=(1, 1))
    )
  )
  (roi_heads): RoIHeads(
    (box_roi_pool): MultiScaleRoIAlign()
    (box_head): TwoMLPHead(
      (fc6): Linear(in_features=50176, out_features=1024, bias=True)
      (fc7): Linear(in_features=1024, out_features=1024, bias=True)
    )
    (box_predictor): FastRCNNPredictor(
      (cls_score): Linear(in_features=1024, out_features=2, bias=True)
      (bbox_pred): Linear(in_features=1024, out_features=8, bias=True)
    )
  )
)

Am I correct? If so, how do I resolve this issue? Is there a STANDARD PRACTICE on dealing with grayscale images and FasterRCNN?
Thanks in advance! Really appreciate it!

Normalize(mean=[0.485, 0.456, 0.406], std=[0.229, 0.224, 0.225]) means the normalization process is applied to all the 3 channels of the input image. 0.485 is applied to the R channel, 0.456 is applied to the G channel and 0.406 is applied to the B channel. The same goes for the standard deviation values.
The 1st Conv. layer of the backbone expects a 1 channel input and that's the reason you get this error.
You could do the following to solve the issue.
Re-define the GeneralizedRCNNTransform and attach it to your model. You could do something like this:
# put the pieces together inside a FasterRCNN model

model = FasterRCNN(backbone, num_classes=2, rpn_anchor_generator=anchor_generator, box_roi_pool=roi_pooler)

# Changes
grcnn = torchvision.models.detection.transform.GeneralizedRCNNTransform(min_size=800, max_size=1333, image_mean=[0.485], image_std=[0.229])
model.transform = grcnn
model.to(device)

