Use new APIs added in PyTorch v1.8.0

[matthewfeickert]

Description

In PyTorch v1.8.0 in the Python API section under "New features" there is the addition of torch.ravel (pytorch/pytorch#46098) and torch.tile (pytorch/pytorch#47974) to the public API. These should be adopted for the torch backend.

Currently we use

pyhf/src/pyhf/tensor/pytorch_backend.py

Lines 194 to 212 in 2c10be3

	def ravel(self, tensor):
	"""
	Return a flattened view of the tensor, not a copy.
	Example:
	>>> import pyhf
	>>> pyhf.set_backend("pytorch")
	>>> tensor = pyhf.tensorlib.astensor([[1.0, 2.0, 3.0], [4.0, 5.0, 6.0]])
	>>> pyhf.tensorlib.ravel(tensor)
	tensor([1., 2., 3., 4., 5., 6.])
	Args:
	tensor (Tensor): Tensor object
	Returns:
	`torch.Tensor`: A flattened array.
	"""
	return tensor.view(-1)

and

pyhf/src/pyhf/tensor/pytorch_backend.py

Lines 126 to 146 in 2c10be3

	def tile(self, tensor_in, repeats):
	"""
	Repeat tensor data along a specific dimension
	Example:
	>>> import pyhf
	>>> pyhf.set_backend("pytorch")
	>>> a = pyhf.tensorlib.astensor([[1.0], [2.0]])
	>>> pyhf.tensorlib.tile(a, (1, 2))
	tensor([[1., 1.],
	[2., 2.]])
	Args:
	tensor_in (:obj:`tensor`): The tensor to be repeated
	repeats (:obj:`tensor`): The tuple of multipliers for each dimension
	Returns:
	PyTorch tensor: The tensor with repeated axes
	"""
	return tensor_in.repeat(repeats)