[BUG] Consistent model naming

[gdevos010]

Describe the bug
NHiTS class should be renamed to NHiTSModel to match naming convention of all other models.

(

darts/darts/models/forecasting/nhits.py

Lines 421 to 434 in 611e411

	class NHiTS(PastCovariatesTorchModel):
	def __init__(
	self,
	input_chunk_length: int,
	output_chunk_length: int,
	num_stacks: int = 3,
	num_blocks: int = 1,
	num_layers: int = 2,
	layer_widths: Union[int, List[int]] = 512,
	pooling_kernel_sizes: Optional[Tuple[Tuple[int]]] = None,
	n_freq_downsample: Optional[Tuple[Tuple[int]]] = None,
	dropout: float = 0.0,
	**kwargs,
	):

)