add r^2 metric (#407)

* add r^2  metric

* Update metrics/r_squared/README.md

Co-authored-by: Leandro von Werra <lvwerra@users.noreply.github.com>

* Update metrics/r_squared/README.md

* Update metrics/r_squared/Readme

* Added r_squared metric

* Update  metrics/r_squared/README.md

* Update r_squared metric

* Update  metrics/r_squared/README.md

* Update metrics/r_squared/app.py

* Update metrics/r_squared/r_squared.py

* Update metrics/r_squared/requirements.txt

* fix: imports and format code according to Black standard

* Fix: R-squared evaluation metric to round off value to 3 decimal places

---------

Co-authored-by: Leandro von Werra <lvwerra@users.noreply.github.com>