pnnx print flops memops count

[luxincn]

qwq

[tencent-adm]

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[luxincn]

包含了大部分pnnx支持的op（torch.nn和torch.nn.functional），如conv，Linear，MultiheadAttention，Pool，LSTM，RNN，Mish等，并进行了简单的测试。
参考了pytorch官方文档定义 (https://pytorch.org/docs/stable/index.html) ，以及其他计算模型FLOPs和内存读取数的库，如
torchstat (https://github.com/Swall0w/torchstat) ，ptflops (https://github.com/sovrasov/flops-counter.pytorch) ，thop (https://github.com/Lyken17/pytorch-OpCounter) 。