no_grad breaks reconstruction tutorial

[eigenvivek]

Need to remove this line

DiffDRR/diffdrr/renderers.py

Lines 52 to 53 in e04e1d8

	with torch.no_grad():
	img = _get_voxel(volume, xyzs, self.mode, align_corners=align_corners)

[eigenvivek]

2D/3D registration (optimizers.ipynb) works with no_grad but 3D reconstruction (reconstruction.ipynb) fails.

no_grad buys an additional 6-7 iterations / second.

Instead, add a flag to toggle the no_grad call.