[WebGL] Benchmark code snippet for conv2d

[Linchenn]

Will not merge this into code base

Will use this code snippet to compare the conv2d's performance of the experimental dense implementation, https://github.com/tensorflow/tfjs/pull/6566, against the current implementation.

Browserstack benchmark

Will test on the following browserstack targets:

• IOS (webgl-1): //tfjs-backend-webgl:bs_ios_11_tfjs-backend-webgl1_test
• Android: //tfjs-backend-webgl:bs_android_9_tfjs-backend-webgl2_test

Example command: yarn bazel test --test_output=streamed --//:grep='Benchmark general conv2d with dense' //tfjs-backend-webgl:bs_android_9_tfjs-backend-webgl2_test --nocache_test_results

Example output:

Local benchmark

And will test locally on Mac, Mac-M1 and Linux. Example command: yarn test --test_verbose_timeout_warnings --verbose_failures --nocache_test_results --//:grep='benchmark dense'

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