Input parameters broken in v23.3

[valentinsulzer]

Discussed in #2852

^{Originally posted by agoshtasbi April 3, 2023}
Hi folks,

It seems that the latest changes have caused some issues with specifying input parameters. The following code snippet (slightly modified from workshop notebooks) used to work in v23.2, but now produces the error below:

`
import pybamm
import numpy as np
model = pybamm.lithium_ion.DFN()
geometry = model.default_geometry
param = model.default_parameter_values

param.update({"Current function [A]": "[input]"})
param.update({"Negative electrode porosity": "[input]"})

param.process_geometry(geometry)
param.process_model(model)

var = pybamm.standard_spatial_vars
var_pts = {var.x_n: 30, var.x_s: 30, var.x_p: 30, var.r_n: 10, var.r_p: 10}
mesh = pybamm.Mesh(geometry, model.default_submesh_types, var_pts)

disc = pybamm.Discretisation(mesh, model.default_spatial_methods)
disc.process_model(model)

t_eval = np.linspace(0, 3600, 100)
solver = pybamm.CasadiSolver(mode="safe", atol=1e-6, rtol=1e-3)

solution = solver.solve(
model, t_eval,
inputs={"Current function [A]": 0.15652, "Negative electrode porosity": 0.3},
calculate_sensitivities=False
)`

pybamm.expression_tree.exceptions.DomainError: Cannot concatenate child 'Column vector of length 30 * Negative electrode porosity' with empty domain

I couldn't find any trace of this in the change log so I'm not sure if the code needs to be changed for v23.3. Any help would be much appreciated.

Thanks!