Process notebooks with pypandoc and a custom pandoc filter

[svekars]

To fix a number of formatting issues with notebooks, adding pypandoc to process the notebooks and a custom pandoc filter and a post-processing script.
The custom filter does the following:

• Handles admonitions - replaces notes, tips, and warnings with correct HTML blocks.
  Before:

• Handles blocks of code that have video frames - adds a tag ``` {.python .jupyter-code-cell}

• Handles images - prepends the pytorch.org/tutorials to the local paths so that the images display correctly.

• Handles 2-colum grid layout - replaces with sid-by side cards:

The post-porocessing script:

• Cleans up the resulting notebooks and remove the unneeded ```{=html} syntax that prevents the notebooks from rendering
• Searches for ``` {.python .jupyter-code-cell} in the generated .ipynb files and slices the cells so that the HTML video frame can be run as a code cell and display that video

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/tutorials/2741

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As of commit 7adf3f6 with merge base 8c48ada ():
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[malfet]

Please do not unpin numpy and tqdm, but rather specify versions that do work for you (all those pins were added when updates broke tutorials)

[malfet]

Left a few comments (most particular, do not unpin the version of dependencies), but overall looks good to me