Fix manual optimization in pl_example

[akihironitta]

What does this PR do?

Follow up of #6294.

This PR makes sure to use:

class Model(pl.LightningModule):
    def __init__(self):
        self.automatic_optimization = False

instead of:

class Model(pl.LightningModule):
    @property
    def automatic_optimization(self):
        return False

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[codecov]

Codecov Report

Merging #6373 (d5aeeeb) into master (4f391bc) will decrease coverage by 2%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #6373    +/-   ##
=======================================
- Coverage      93%     91%    -2%     
=======================================
  Files         161     161            
  Lines       11464   11464            
=======================================
- Hits        10706   10483   -223     
- Misses        758     981   +223