Add cpu compatibility info in documentation (#4405)

* Add cpu compatibility info

* Add table about not support CPU only model info

* Update docs about not support CPU only model info

docs/get_started.md

@@ -118,12 +118,28 @@ In CPU mode you can run the demo/webcam_demo.py for example.
 However some functionality is gone in this mode:
 
 - Deformable Convolution
+- Modulated Deformable Convolution
+- ROI pooling
 - Deformable ROI pooling
 - CARAFE: Content-Aware ReAssembly of FEatures
+- SyncBatchNorm
+- CrissCrossAttention: Criss-Cross Attention
+- MaskedConv2d
+- Temporal Interlace Shift
 - nms_cuda
 - sigmoid_focal_loss_cuda
+- bbox_overlaps
 
-So if you try to run inference with a model containing deformable convolution you will get an error.
+So if you try to run inference with a model containing above ops you will get an error. The following table lists the related methods that cannot inference on CPU due to dependency on these operators
+
+|                        Operator                         |                            Model                             |
+| :-----------------------------------------------------: | :----------------------------------------------------------: |
+| Deformable Convolution/Modulated Deformable Convolution | DCN、Guided Anchoring、RepPoints、CentripetalNet、VFNet、CascadeRPN、NAS-FCOS、DetectoRS |
+|                      MaskedConv2d                       |                       Guided Anchoring                       |
+|                         CARAFE                          |                            CARAFE                            |
+|                      SyncBatchNorm                      |                           ResNeSt                            |
+
+**Notice**: MMDetection does not support training with CPU for now.
 
 ### Another option: Docker Image