Add grad compute for ThresholdedReLUOp, LeakyReluOp

[pcpLiu]

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DO NOT Submit Non-bug PR Here

Please go to pcpLiu/SerranoExplore to discuss feature enhancements. This repo only accepts bug-related PRs.

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Description

Add grad compute for ThresholdedReLUOp, LeakyReluOp

Related Issues

N/A

How Has This Been Tested?

Unit Test

[codecov-io]

Codecov Report

Merging #8 into master will decrease coverage by 37.86%.
The diff coverage is 25.6%.

@@             Coverage Diff             @@
##           master       #8       +/-   ##
===========================================
- Coverage   73.59%   35.73%   -37.87%     
===========================================
  Files         133      133               
  Lines       18386    17259     -1127     
  Branches      750      366      -384     
===========================================
- Hits        13532     6168     -7364     
- Misses       4789    11056     +6267     
+ Partials       65       35       -30

Impacted Files	Coverage Δ
...ranoTests/op/nn/activation/leakyrelu_op_test.swift	0% <0%> (-81.09%)	⬇️
...errano/operators/nn/activation/activation_op.swift	16.84% <12.59%> (-39.28%)	⬇️
...sts/op/nn/activation/thresholdedrelu_op_test.swift	92.5% <92.5%> (+14.12%)	⬆️
Examples/Graph/VGG16.swift	0% <0%> (-100%)	⬇️
Examples/Graph/SimpleCNN.swift	0% <0%> (-100%)	⬇️
...sts/op/basic/broadcast/broadcast_add_op_test.swift	0% <0%> (-100%)	⬇️
...sts/op/basic/broadcast/broadcast_sub_op_test.swift	0% <0%> (-100%)	⬇️
Tests/SerranoMacTests/SerranoMacTests.swift	0% <0%> (-100%)	⬇️
...ts/op/basic/broadcast/broadcast_mult_op_test.swift	0% <0%> (-100%)	⬇️
Examples/Imperative/SimpleCNN_imperative.swift	0% <0%> (-100%)	⬇️
... and 95 more

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