`mc` model config overwritten by `instantiate_*` makes it unusable.

[AzulGarza]

After getting the best hyperopt trial using nf.experiments.utils.hyperopt_tunning, I wanted to use the mc provided in the trial results in nf.experiments.utils.instantiate_nbeats. I got the following error:

The problem arises from these lines:

https://github.com/Nixtla/neuralforecast/blob/09ce6d0075bbf287d78483d04f107dec7505fc5c/neuralforecast/experiments/utils.py#L270-L273

Initially, n_blocks and n_layers are integers (denoting the number of blocks and layers). But later, nf.experiments.utils.instantiate_nbeats converts them to a list of integers. Therefore, the mc dictionary becomes unusable for reuse by nf.experiments.utils.instantiate_nbeats.

[AzulGarza]

Since n_layers and n_blocks are parameters of the model, I think the best solution is to rename the hyperopt space parameters to constant_n_layers and constant_n_blocks,

https://github.com/Nixtla/neuralforecast/blob/09ce6d0075bbf287d78483d04f107dec7505fc5c/neuralforecast/models/nbeats/nbeats.py#L827-L828

[AzulGarza]

The same applies for n_mlp_units.