Support command line params merging with .yaml

[Alicegaz]

Specifying parameters through the command line works only if those parameters are in a config file used to run the train.py. So merging only works in a case of override, but not in the case of a union of .yaml config parameters and parameters from the command line.
Way to reproduce the issue:

python train.py -cp examples/configs -cn classification_cifar10 trainer.accelerator='cpu' task.params.freeze_backbone=true

Error Message:

Could not override 'task.params.freeze_backbone'.
To append to your config use +task.params.freeze_backbone=true
Key 'freeze_backbone' is not in struct
    full_key: task.params.freeze_backbone
    object_type=dict

Set the environment variable HYDRA_FULL_ERROR=1 for a complete stack trace.