Memory leakage in long term forecast

**Description:**
I encountered a memory leakage issue when using a large dataset for long-term forecasting. It seems the problem occurred because the loss was being accumulated as a tensor instead of a scalar.

**Solution:**
To fix this, I changed:
`loss_val += loss` to `loss_val += loss.item()`
at the following lines:

- exp/exp_long_term_forecasting.py#144

- exp/exp_long_term_forecasting.py#149
