class_docstr: Blur a tensor and downsamples it.
  
  Args:
      border_type: the padding mode to be applied before convolving.
        The expected modes are: ``'constant'``, ``'reflect'``,
        ``'replicate'`` or ``'circular'``.
      align_corners: interpolation flag.
      factor: the downsampling factor
  
  Return:
      the downsampled tensor.
  
  Shape:
      - Input: :math:`(B, C, H, W)`
      - Output: :math:`(B, C, H / 2, W / 2)`
  
  Examples:
      >>> input = torch.rand(1, 2, 4, 4)
      >>> output = PyrDown()(input)  # 1x2x2x2
class_docstr_tok: ["Blur","a","tensor","and","downsamples","it",".","Args",":","border_type",":","the","padding","mode","to","be","applied","before","convolving",".","The","expected","modes","are",":","``","'constant","'","``",",","``","'reflect","'","``",",","``","'replicate","'","``","or","``","'circular","'","``",".","align_corners",":","interpolation","flag",".","factor",":","the","downsampling","factor","Return",":","the","downsampled","tensor",".","Shape",":","-","Input",":",":","math",":","`","(","B",",","C",",","H",",","W",")","`","-","Output",":",":","math",":","`","(","B",",","C",",","H","\/","2",",","W","\/","2",")","`","Examples",":",">",">",">","input","=","torch.rand","(","1",",","2",",","4",",","4",")",">",">",">","output","=","PyrDown","(",")","(","input",")","#","1x2x2x2"]
code: def __init__(
      self,
      border_type: str = "reflect",
      align_corners: bool = False,
      factor: float = 2.0,
  ) -> None:
      super().__init__()
      self.border_type: str = border_type
      self.align_corners: bool = align_corners
      self.factor: float = factor
  
imports: from __future__ import annotations
  import math
  import torch
  import torch.nn.functional
  from kornia.core import Module, Tensor, ones, pad, stack, tensor, zeros
  from kornia.core.check import KORNIA_CHECK, KORNIA_CHECK_IS_TENSOR, KORNIA_CHECK_SHAPE
  from kornia.filters import filter2d, gaussian_blur2d
label_desc: Use image node_id 1 to create a new PyrDown object from inherited base classes: Module with example: obj = PyrDown(border_type, align_corners, factor)