Use official emsdk bazel toolchain

[mattsoulanille]

Build tfjs-backend-wasm with the official emsdk toolchain open-sourced from google3.

Using the official toolchain fixes the issue where normal, simd, and simd + threaded builds write their artifacts to the same output location, invalidating the build cache. With this change, outputs are created in separate folders, enabling build caching.

Additionally, the emscripten toolchain is now automatically downloaded as a dependency within Bazel, so it no longer needs to be installed separately.

A minor edit to the official toolchain was needed to make simd work correctly with XNNPACK.

This PR also enables remote build caching for the wasm backend on CI runs.

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This change is 

[mattsoulanille]

Remote caching is not working in GCP yet, but is working with a locally run 'remote' cache. I'm sending this out for review now, but we might want to delay merging it until after the remote cache is working on GCP (which I'm pretty sure just requires some GCP config changes but might require some changes to this PR as well).

[mattsoulanille]

Note to reviewers: Everything in emscripten_toolchain/ is from the emsdk Bazel toolchain with a minor edit. Also, the new toolchain required a few changes to the cc source files, mostly involving removing unused variables. These are detailed in remove unused variables.

[jinjingforever]

This is super cool! Thank you so much for making this work!

Reviewed 6 of 34 files at r1.
Reviewable status: complete! 1 of 1 approvals obtained (waiting on @lina128, @mattsoulanille, and @pyu10055)

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.bazelrc, line 23 at r2 (raw file):

to so

nit: typo

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emscripten_toolchain/emar.sh, line 1 at r1 (raw file):

#!/bin/bash

Do we need to add the license text to these files?

[pyu10055]

Thanks Matt, this is great. A high level question, is your change to the toolchain upstreamed to the repo? should we maintain a copy of the emscripten_toolchain files or be downloaded as needed from the repo?

Reviewable status: complete! 1 of 1 approvals obtained (waiting on @lina128 and @mattsoulanille)

[mattsoulanille]

My change isn't upstreamed since I'm pretty sure it's specific to just xnnpack. We should probably maintain our own copy of emscripten_toolchain, but we could set up a copybara to automatically pull new changes and apply the changes I made.

I also changed emscripten's npm_install name to avoid a conflict with our monorepo's yarn_install, so we'd need to use copybara even if the change were upstreamed.

Reviewable status: complete! 1 of 1 approvals obtained (waiting on @jinjingforever and @lina128)

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.bazelrc, line 23 at r2 (raw file):

Previously, jinjingforever (Jing Jin) wrote…

to so

nit: typo

Thanks!

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emscripten_toolchain/emar.sh, line 1 at r1 (raw file):

Previously, jinjingforever (Jing Jin) wrote…

Do we need to add the license text to these files?

I don't think so since they're pulled directly from emsdk per their setup instructions.

[mattsoulanille]

I tried this on my mac, and I think this PR may break support for compiling wasm on OSX and Windows. It looks like the build rules only ship with Linux binaries for emscripten. I'll check with the author.

Edit: looks like there's an issue open for this: emscripten-core/emsdk#650

[lina128]

Great work!!! Just curious, if we are going to sync wasm backend to g3 and allows it to build in g3, do we also use these toolchain?

Reviewable status: complete! 2 of 1 approvals obtained (waiting on @jinjingforever)

[mattsoulanille]

I think we can build in g3 using g3 rules. They're very similar to the open source ones. I'll give it a try.

Reviewable status: complete! 2 of 1 approvals obtained (waiting on @jinjingforever)

[mattsoulanille]

Support for operating systems other than linux has been fixed in emsdk, and the way the toolchain is loaded has been significantly simplified (it's now loaded as an external repository instead of copied into our workspace).

I've tested this PR on my mac again, and it now compiles and passes the tests. I think it's ready to be merged, but PTAL again. Thanks!

[pyu10055]

Thanks! Cannot wait to see it in action.

[lina128]

Great work, thank you Matt!