Fix bug with max_steps in modelPT.py according to PTL 1.5.0
Fix bug in modelPT.py according to this PR in PTL 1.5.0: 
https://github.com/PyTorchLightning/pytorch-lightning/commit/0e20119d24c410c9004a9d0c6ce0505914c1dc02

Signed-off-by: Oktai Tatanov <oktai.tatanov@gmail.com>