fix sh l=0

[mariogeiger]

Since now cuequivariance_ops_torch.SymmetricTensorContraction supports STP of degree 0 we can simplify the code by removing the special case for degree 0 in cuet.SymmetricTensorProduct

import cuequivariance_ops_torch as ops
import torch

f = ops.SymmetricTensorContraction(
    [
        [0, 0],  # degree 0 (was not supported before)
        [0, 0, 1],  # degree 1
        [0, 0, 0, 2],  # degree 2
    ],
    [
        5.0,
        1.0,
        1.0,
    ],
    num_in_segments=1,
    num_couplings=1,
    num_out_segments=3,
    correlation=2,
    math_dtype=torch.float32,
).to(device="cuda")
f(
    torch.tensor([[[2.0]]], device="cuda"),  # input
    torch.tensor([[[2.0]]], device="cuda"),  # coupling
    torch.tensor([0], device="cuda", dtype=torch.int32),
)

[moradza]

LGTM!