Unexpected behaviour of hooking inside callback
🐛 Bug
on_epoch_end hook in callback called between train and validation epochs.
To Reproduce
Use following callback
class BuggyCallback(Callback):
    def on_epoch_end(self, trainer: Trainer, pl_module: LightningModule):
        print("I'm called")

Expected behavior
Line with I'm called should appear after train and validation cycles. Instead of this, that line printed exactly after the training cycle, before validation.
Environment

PyTorch Version (e.g., 1.0): 1.7.1
OS (e.g., Linux): MacOS BigSur
How you installed PyTorch (conda, pip, source): conda
Build command you used (if compiling from source): -
Python version: 3.8.5
CUDA/cuDNN version: CPU version
GPU models and configuration: -
Any other relevant information: -

Additional context
I checked manually that it works properly in the 1.1.7 version. Thus, the bug appears only in the last version of lightning.