class_docstr: Helper class for KeyNet.
  
  It defines the learnable blocks within the Key.Net
code: def forward(self, x: Tensor) -> Tensor:
      x = self.conv2(self.conv1(self.conv0(x)))
      return x
  
imports: from typing import List, Optional
  import torch
  import torch.nn.functional
  from torch import nn
  from typing_extensions import TypedDict
  from kornia.core import Module, Tensor, concatenate
  from kornia.filters import SpatialGradient
  from kornia.geometry.transform import pyrdown
  from kornia.utils.helpers import map_location_to_cpu
  from .scale_space_detector import Detector_config, MultiResolutionDetector, get_default_detector_config
label_desc: Use image node_id 2 for calling the _LearnableBlock obj's underlying member method code with example usage: obj.forward(x) and returns: x