Enhance readme for ddp cases in ldm tutorials

[KumoLiu]

Enhance readme for ddp cases in ldm tutorials
Add amp argument in maisi diffusion training notebook as a workaround for #1858.

Checks

• Avoid including large-size files in the PR.
• Clean up long text outputs from code cells in the notebook.
• For security purposes, please check the contents and remove any sensitive info such as user names and private key.
• Ensure (1) hyperlinks and markdown anchors are working (2) use relative paths for tutorial repo files (3) put figure and graphs in the ./figure folder
• Notebook runs automatically ./runner.sh -t <path to .ipynb file>

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[dongyang0122]

Thank you. All changes look good to me.