Move adaptive avg pooling 2d to ATen native (#14714)

Move adaptive avg pooling 2d to ATen native (#14714)

Summary:
adaptive_avg_pool1d, adaptive_avg_pool2d, and adaptive_avgpool3d are neural network functions that are currently implemented in our legacy THNN (CPU) / THCUNN (CUDA) libraries.  It is generally better if these live in our new library ATen, since it is more feature complete and reduces cognitive overhead.

This change moves currently to adaptive_avg_pool1d and adaptive_avg_pool2d to ATen.

timed relevant cpu tests with this change:
```
[ialex@devgpu064.ash5 ~/pytorch] time python test/test_nn.py
test_AdaptiveAvgPool1d (__main__.TestNN)
test_AdaptiveAvgPool1d_cuda (__main__.TestNN)
test_AdaptiveAvgPool2d_single (__main__.TestNN)
test_AdaptiveAvgPool2d_single_cuda (__main__.TestNN)
test_AdaptiveAvgPool2d_tuple (__main__.TestNN)
test_AdaptiveAvgPool2d_tuple_cuda (__main__.TestNN)
test_AdaptiveAvgPool2d_tuple_none (__main__.TestNN)
test_AdaptiveAvgPool2d_tuple_none_cuda (__main__.TestNN)
test_AdaptiveAvgPool3d_single (__main__.TestNN)
test_AdaptiveAvgPool3d_single_cuda (__main__.TestNN)
test_AdaptiveAvgPool3d_tuple (__main__.TestNN)
test_AdaptiveAvgPool3d_tuple_cuda (__main__.TestNN)
test_AdaptiveAvgPool3d_tuple_none (__main__.TestNN)
test_AdaptiveAvgPool3d_tuple_none_cuda (__main__.TestNN)
test_adaptive_log_softmax (__main__.TestNN)
test_adaptive_pooling_input_size (__main__.TestNN)
test_adaptive_pooling_size_none (__main__.TestNN)
.s.s.s.s.s.s.s...
----------------------------------------------------------------------
Ran 17 tests in 6.273s

OK (skipped=7)

real	0m7.164s
user	3m1.289s
sys	0m0.905s
```

compared to master:
```
[ialex@devgpu064.ash5 ~/pytorch] time python test/test_nn.py
test_AdaptiveAvgPool1d (__main__.TestNN)
test_AdaptiveAvgPool1d_cuda (__main__.TestNN)
test_AdaptiveAvgPool2d_single (__main__.TestNN)
test_AdaptiveAvgPool2d_single_cuda (__main__.TestNN)
test_AdaptiveAvgPool2d_tuple (__main__.TestNN)
test_AdaptiveAvgPool2d_tuple_cuda (__main__.TestNN)
test_AdaptiveAvgPool2d_tuple_none (__main__.TestNN)
test_AdaptiveAvgPool2d_tuple_none_cuda (__main__.TestNN)
test_AdaptiveAvgPool3d_single (__main__.TestNN)
test_AdaptiveAvgPool3d_single_cuda (__main__.TestNN)
test_AdaptiveAvgPool3d_tuple (__main__.TestNN)
test_AdaptiveAvgPool3d_tuple_cuda (__main__.TestNN)
test_AdaptiveAvgPool3d_tuple_none (__main__.TestNN)
test_AdaptiveAvgPool3d_tuple_none_cuda (__main__.TestNN)
test_adaptive_log_softmax (__main__.TestNN)
test_adaptive_pooling_input_size (__main__.TestNN)
test_adaptive_pooling_size_none (__main__.TestNN)
.s.s.s.s.s.s.s...
----------------------------------------------------------------------
Ran 17 tests in 7.232s

OK (skipped=7)

real	0m8.065s
user	3m34.714s
sys	0m2.440s
```

also timed relevant cuda tests with this change:
```
[ialex@devgpu064.ash5 ~/pytorch] time python test/test_nn.py
test_AdaptiveAvgPool1d (__main__.TestNN)
test_AdaptiveAvgPool1d_cuda (__main__.TestNN)
test_AdaptiveAvgPool2d_single (__main__.TestNN)
test_AdaptiveAvgPool2d_single_cuda (__main__.TestNN)
test_AdaptiveAvgPool2d_tuple (__main__.TestNN)
test_AdaptiveAvgPool2d_tuple_cuda (__main__.TestNN)
test_AdaptiveAvgPool2d_tuple_none (__main__.TestNN)
test_AdaptiveAvgPool2d_tuple_none_cuda (__main__.TestNN)
test_AdaptiveAvgPool3d_single (__main__.TestNN)
test_AdaptiveAvgPool3d_single_cuda (__main__.TestNN)
test_AdaptiveAvgPool3d_tuple (__main__.TestNN)
test_AdaptiveAvgPool3d_tuple_cuda (__main__.TestNN)
test_AdaptiveAvgPool3d_tuple_none (__main__.TestNN)
test_AdaptiveAvgPool3d_tuple_none_cuda (__main__.TestNN)
test_adaptive_log_softmax (__main__.TestNN)
test_adaptive_pooling_input_size (__main__.TestNN)
test_adaptive_pooling_size_none (__main__.TestNN)
.................
----------------------------------------------------------------------
Ran 17 tests in 21.049s

OK

real	0m24.106s
user	0m20.890s
sys	0m4.026s
```

compared to master
```
[ialex@devgpu064.ash5 ~/pytorch] time python test/test_nn.py
test_AdaptiveAvgPool1d (__main__.TestNN)
test_AdaptiveAvgPool1d_cuda (__main__.TestNN)
test_AdaptiveAvgPool2d_single (__main__.TestNN)
test_AdaptiveAvgPool2d_single_cuda (__main__.TestNN)
test_AdaptiveAvgPool2d_tuple (__main__.TestNN)
test_AdaptiveAvgPool2d_tuple_cuda (__main__.TestNN)
test_AdaptiveAvgPool2d_tuple_none (__main__.TestNN)
test_AdaptiveAvgPool2d_tuple_none_cuda (__main__.TestNN)
test_AdaptiveAvgPool3d_single (__main__.TestNN)
test_AdaptiveAvgPool3d_single_cuda (__main__.TestNN)
test_AdaptiveAvgPool3d_tuple (__main__.TestNN)
test_AdaptiveAvgPool3d_tuple_cuda (__main__.TestNN)
test_AdaptiveAvgPool3d_tuple_none (__main__.TestNN)
test_AdaptiveAvgPool3d_tuple_none_cuda (__main__.TestNN)
test_adaptive_log_softmax (__main__.TestNN)
test_adaptive_pooling_input_size (__main__.TestNN)
test_adaptive_pooling_size_none (__main__.TestNN)
.................
----------------------------------------------------------------------
Ran 17 tests in 23.021s

OK

real	0m27.095s
user	0m20.121s
sys	0m3.668s
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/14714

Differential Revision: D13384084

Pulled By: xnder

fbshipit-source-id: 344442103ccbbda72d3c010d2feea00e9985d226