Tutorials bugs to improve

[binliunls]

Describe the bug
There are some bugs found when tried to run these tutorials below.

• Reconstruction tutorial:

  • some typo and grammar problems in the readme file.
  • fastmri_ssim.py absents in varnet_demo folder.
  • the checkpoint device directly map to 'cpu' in unet_demo/inference.ipynb.

• Detection tutorial:

  • some typo, link and grammar problems in the readme file.

• CAI tutorial:

  • add explanation for opencv codec error in the video_seg tutorial.
  • use EnsureChannelFirst instead of AsChannelFirst in endoscopic inbody classification tutorial.
  • accuracy in endoscopic inbody classfication tutorial should be a number instead of tensor.

• Pathology tutorial:

  • some useless cell output in nuclick tutorial.

• DeepAtlas tutorial:

  • the print output run by myself is different from the original one in github.