Added functionality to calculate the diameter of given binary tree (#10526)

* Added code to find diameter of given binary tree

* Modified diameter_of_binary_tree file

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* Update diameter_of_binary_tree.py

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* Update diameter_of_binary_tree.py

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Christian Clauss <cclauss@me.com>