Add test for DeepNPTS

[rshyamsundar]

Issue #, if available:

Description of changes:

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[lostella]

I think this was moved:

gluonts/test/torch/model/test_estimators.py

Lines 97 to 104 in 43e78d3

	lambda dataset: DeepNPTSEstimator(
	freq=dataset.metadata.freq,
	prediction_length=dataset.metadata.prediction_length,
	context_length=2 * dataset.metadata.prediction_length,
	batch_size=4,
	num_batches_per_epoch=3,
	epochs=2,
	),