Fix cuDnn build error with CC3.0 platform(#25820) (#25825)
Summary:
__ldg is only available for CC3.5 and above, add default implementation for CC3.0 platform.
This PR along with jcooky's PR of https://github.com/jcooky/pytorch/commit/ecdf4564d44835b3b2ffd18e286ad7e549231a14. make the pytorch master HEAD build and runs properly for CC3.0 platform(such as Retina MacBook Pro of Late 2013).
I test the mnist example from pytorch/examples with the wheel built, the test accuracy ends with 99% after 10 Epochs with GT 750M CC3.0 platform. CC3.0 platform decrease training time into about 1/5 of its cpu counterpart.
```
(pytorch) SamuelFdeMBP:mnist sfeng$ pip list | grep torch
pytorch-sphinx-theme 0.0.24 /Users/sfeng/GH/pytorch_110/docs/src/pytorch-sphinx-theme
torch 1.3.0a0+a332583
torchvision 0.5.0a0+0bd7080
(pytorch) SamuelFdeMBP:mnist sfeng$ date && time python main.py && date
日 9 8 07:17:38 CST 2019
/Users/sfeng/anaconda3/envs/pytorch/lib/python3.7/site-packages/torch/cuda/__init__.py:132: UserWarning:
Found GPU0 GeForce GT 750M which is of cuda capability 3.0.
PyTorch no longer supports this GPU because it is too old.
The minimum cuda capability that we support is 3.5.
warnings.warn(old_gpu_warn % (d, name, major, capability[1]))
Train Epoch: 1 [0/60000 (0%)] Loss: 2.300039
......
Train Epoch: 10 [59520/60000 (99%)] Loss: 0.007440
Test set: Average loss: 0.0322, Accuracy: 9895/10000 (99%)
real 2m39.962s
user 4m13.625s
sys 0m9.672s
日 9 8 07:20:18 CST 2019
(pytorch) SamuelFdeMBP:mnist sfeng$ date && time python main.py --no-cuda && date
日 9 8 07:20:40 CST 2019
Train Epoch: 1 [0/60000 (0%)] Loss: 2.300039
Train Epoch: 1 [640/60000 (1%)] Loss: 2.213470
Train Epoch: 1 [1280/60000 (2%)] Loss: 2.170460
......
Train Epoch: 10 [58880/60000 (98%)] Loss: 0.005681
Train Epoch: 10 [59520/60000 (99%)] Loss: 0.007686
Test set: Average loss: 0.0319, Accuracy: 9894/10000 (99%)
real 12m6.604s
user 75m53.129s
sys 3m41.744s
日 9 8 07:32:47 CST 2019
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25825
Differential Revision: D17252176
Pulled By: soumith
fbshipit-source-id: 70bf84ae6380be86b56344b161a52fb06a53a1b2
