MNN对大分辨率输入的GPU(OpenCL)加速效果不明显（网络包含较多的conv层）

[pedroHuang123]

Platform(Include target platform as well if cross-compiling):

Windows
分别采用CPU和GPU测试，时间为8s和6s，加速效果不明显，是否和分辨率过大相关，或者和使用了Attention结构相关？

Build Log:

Open Model .\mwispnet_train_kdv4_demosaic_wb_2784_2784.mnn
The device support i8sdot:0, support fp16:0, support i8mm: 0
test_main, 282, cost time: 785.259033 ms
Session Info: memory use 1859.915771 MB, flops is 763125.750000 M, backendType is 13
===========> Session Resize Done.
===========> Session Start running...
Input size:31002624
Tensor shape: 1, 3, 2784, 2784,
fileName.str().c_str()=s ./input_0.txt in _loadInputFromFile, 110
output: output
precision:2, memory: 0, Run 10 time:
详细log如下:
CPU_log.txt
OpenCL_log.txt

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