* WIP

* More WIP

* More chiseling

* Move conv validation utils to a separate file; update unit tests

* Fix invalid attributes in pattern containing ConvolutionBackpropFilters

* Remove zero_const_conv test (it's no longer possible to construct the graph being tested)

* Rename infer_convolution_output_item_shape to infer_windowed_reduction_output_shape and add a boolean flag to control whether window-all-in-padding is allowed

* Add generalized function for inferring pooling fprop, use it in AvgPool/AvgPoolBackprop

* Update MaxPool to use new utility functions

* Fix comment

* Remove faulty and redundant check for window shape relative to pre-padding data shape

* Revert change to pattern construction in cpu_fusion

* Update unit test for maxpool

* Restore unjustly eliminated tests; move some computation to ptrdiff_t for safety; fix wording on some error messages

* Formatting