fix: Fixed support of modules that are used multiple times during forward

[frgfm]

This PR handles the cases where a single module is reused multiple times. The issue was that with the initial hooking mechanism, the module gets hooked N times, and upon the forward pass, it will trigger N hooks each time the module is used. This was handled by:

• setting a hook tracker for each hook type
• also added support of torch.nn.Flatten
• updating unittests
• fixed requirements

[codecov[bot]]

Codecov Report

Merging #24 into master will decrease coverage by 0.28%. The diff coverage is 82.27%.

@@            Coverage Diff             @@
##           master      #24      +/-   ##
==========================================
- Coverage   89.75%   89.46%   -0.29%     
==========================================
  Files           9        9              
  Lines         527      560      +33     
==========================================
+ Hits          473      501      +28     
- Misses         54       59       +5     

Impacted Files	Coverage Δ
torchscan/crawler.py	82.08% <80.55%> (+0.18%)	:arrow_up:
torchscan/modules/flops.py	99.01% <100.00%> (ø)
torchscan/modules/macs.py	100.00% <100.00%> (ø)
torchscan/modules/memory.py	100.00% <100.00%> (ø)
torchscan/modules/receptive.py	100.00% <100.00%> (ø)