[Vectorised branch] Tracking crashes with single `Marker`, or only elements without `lengths` between non skippable elemtents

[cr-xu]

Currently, the Segment tracking will crash if it only contains.

Since it groups skippable elements in Segment and calls track recursively, it further causes problem if there are only length-less elements between non-skippable elements.

Reproducing the bug:

import cheetah
import torch

beam_in = cheetah.ParticleBeam.from_parameters(num_particles=100)

# Only Marker
segment1 = cheetah.Segment([cheetah.Marker(name="start")])

# This fails
beam_out = segment1.track(beam_in)

# Only length-less elements between non-skippable elements
segment2 = cheetah.Segment(
    [
        cheetah.Cavity(
            length=torch.tensor([0.1]), voltage=torch.tensor([1e6]), name="C2"
        ),
        cheetah.Marker(name="start"),
        cheetah.Cavity(
            length=torch.tensor([0.1]), voltage=torch.tensor([1e6]), name="C1"
        ),  # non-skippable
    ]
)

# This fails too
beam_out = segment2.track(beam_in)

# Only skippable elements
segment3 = cheetah.Segment(
    [
        cheetah.Marker(name="start"),
        cheetah.Drift(length=torch.tensor([0.1]), name="D1"),
        cheetah.Quadrupole(
            length=torch.tensor([0.1]), k1=torch.tensor([1.0]), name="Q1"
        ),
        cheetah.Marker(name="end"),
    ]
)

# This works
beam_out = segment3.track(beam_in)

This will complain:

  File ".../cheetah/accelerator.py", line 2219, in length
    lengths = torch.stack(
RuntimeError: stack expects a non-empty TensorList

This happens only on the vectorised branch, and should be fixed before merging #116

[cr-xu]

Hmm... I guess this is a general issue w.r.t. vectorised branch as the Marker doesn't get broadcasted along with other elements. The transfer map is not properly determined because it doesn't have a length attribute.

One easy fix would be to just assign length as a basic feature in the Element class and set it to 0.0.

What's your opinion @jank324

[jank324]

I think setting length to 0.0 makes sense. On a base Element (and all that don't override length this should probably also be a read-only property.

We might have to check then where exceptions have been made for lengthless elements, code may potentially be simplified.

[cr-xu]

Are you working on this or should I do it?

[jank324]

If you could do it, that would be great!