Calculators fail to `torch.jit.load`

[frostedoyster]

For example, this fails:

from torchpme.calculators import EwaldCalculator
from torchpme import InversePowerLawPotential
import torch


class Model(torch.nn.Module):
    def __init__(self):
        super().__init__()
        self.calculator = EwaldCalculator(
            potential=InversePowerLawPotential(
                exponent=1,
                smearing=1.0,
                exclusion_radius=5.0,
            ),
            full_neighbor_list=True,
            lr_wavelength=5.0,
            prefactor=1.0,
        )

    def forward(self, x):
        return self.calculator(x, x, x, x, x)

tensor = torch.randn(10, 3)

model = Model()
model = torch.jit.script(model)
torch.jit.save(model, "model.pt")
loaded_model = torch.jit.load("model.pt")

Sorry for the unrealistic example, but the same also happens when feeding appropriate tensors to the model.

The error:

Traceback (most recent call last):
  File ".../program.py", line 28, in <module>
    torch.jit.save(model, "model.pt")
  File ".../lib/python3.12/site-packages/torch/jit/_serialization.py", line 84, in save
    m.save(f, _extra_files=_extra_files)
  File ".../lib/python3.12/site-packages/torch/jit/_script.py", line 754, in save
    return self._c.save(str(f), **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
RuntimeError: 
Could not export Python function call '_CustomExp1'. Remove calls to Python functions before export. Did you forget to add @script or @script_method annotation? If this is a nn.ModuleList, add it to __constants__:
  File ".../lib/python3.12/site-packages/torchpme/lib/math.py", line 76
    :return: Exponential integral E1(x)
    """
    return _CustomExp1.apply(x)
           ~~~~~~~~~~~~~~~~~ <--- HERE

[E-Rum]

Good catch, @frostedoyster, thank you very much! After a bit of research, I found that, unfortunately, torch.script does not support custom autograd functions. We need to figure out how to replace it in this case.