Fixed Hydra-core configuration
Hydra-core version 1.1.0  was released on 11th June 2021, and it requires OmegaConf == 2.1.\*, while Nemo requires OmegaConf >=2.0.5 & <2.1.*. Therefore, the existing tutorial notebooks( can be found [here](https://docs.nvidia.com/deeplearning/nemo/user-guide/docs/en/main/starthere/tutorials.html), on importing the models, reported a configuration error. This pull request changed the specified version of hydra-core, specified in the requirements, from >=1.0.4 to ==1.0.6, which supports OmegaConf <2.1.0, and the tutorial notebooks run correctly, without any error