BugFix: fix DAG doc display (especially for TaskFlow DAGs)

[XD-DENG]

Issue and Background

Let's take the example DAG tutorial_taskflow_api_etl as example. In 2.0.1, the DAG doc in Markdown format is not rendered properly.

If it's rendered properly, it should look like the DAG Docs of another example DAG, tutorial,

Why

Because of how TaskFlow DAGs are constructed, their __doc__ lines may start with spaces. This fails markdown.markdown(), and the doc in Markdown format cannot be transformed into HTML properly, and further fails the doc display in the UI.

Actually this also affects the non-TaskFlow DAGs (if users accidentally add a space in the beginning of any line in the __doc__)

def x(x):
    """
    ### Header
    XD's test
    2nd line
    """
    return x * 2

import markdown

print('Case-1')
print(markdown.markdown(x.__doc__))

print('Case-2')
print(markdown.markdown('\n'.join(line.lstrip() for line in x.__doc__.split('\n'))))

Output:

Case-1
<pre><code>### Header
XD's test
2nd line
</code></pre>

Case-2
<h3>Header</h3>
<p>XD's test
2nd line</p>

Solution

This commit fixes this by always doing left strip for each line of the doc md.

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[Richiecakes]

I believe this commit prevents the use of e.g nested lists in markdown docs where indentation is important. Does whitespace/tabs need to be stripped from every line of all markdown docstrings?