Missing Pytorch operators nn.MaxPool2d and nn.MaxUnpool2d

_- Name of layer type:_ MaxPool2d, MaxUnpool2d
_- Is this a PyTorch or a TensorFlow layer type:_ Pytorch
_- Your version of coremltools:_ 5.2.0
_- Your version of PyTorch/TensorFlow:_ 1.11
_- Impact of supporting this layer type. Why is adding support for this layer type important? Is it necessary to support a popular model or use case?_
Nowadays, a lot of models use these layers, especially encoder/decoder ones. Recent state-of-the-art models cannot be converted to CoreML due to the lack of support for these layers

The issue names the PyTorch MaxPool2d and MaxUnpool2d layer types but mentions no files, tests, or entry points. Start by locating the PyTorch operator-conversion support and existing pooling tests; done means models using both layers convert successfully to Core ML.

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