Upgrade @tensorflow/tfjs to 1.2.3 and handle strings as bytes

[nkreeger]

This PR will re-work string usage in the tfjs-node C++ binding code. Currently, the binding reads out string values as string values (char*) and fits them into a heap-based buffer (avoid stack thrashing).

With the change in tfjs-core, the buffers are already managed by V8 so need to manage a heap buffer. However, strings tensors must be encoded using the appropriate TF APIs. This change handles writing uint8 array buffers into TF Tensors, and re-rendering them into typed arrays before surfacing to JS land.

I don't think unit tests will pass here until tfjs-union is published.

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This change is 

[dsmilkov]

Reviewed 2 of 3 files at r1.
Reviewable status: complete! 1 of 1 approvals obtained (waiting on @dsmilkov and @nkreeger)

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binding/tfjs_backend.cc, line 186 at r1 (raw file):

// Creates a TFE_TensorHandle from a JS array of Uint8Array values. TFE_TensorHandle *CreateTFE_TensorHandleFromUtf8StringArray(

the underlying bytes behind the Uint8Array[] could have any encoding so the fromStringArray suffix in the method name is more correct.

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binding/tfjs_backend.cc, line 214 at r1 (raw file):

 if (array_type != napi_uint8_array) { NAPI_THROW_ERROR(env, "Unsupported array type - expecting Uint8 typed array");

nit: say Uint8Array to be more precise.

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binding/tfjs_backend.cc, line 247 at r1 (raw file):

 ENSURE_TF_OK_RETVAL(env, tf_status, nullptr); offsets[i] = cur_str_data - str_data_start;

it's a bummer that the TF C API doesn't provide better utilities for constructing string tensors. Looks like their internal storage details are leaking to the user.

[nkreeger]

@dsmilkov PTAL - updated to 1.2.3 and resolved comments.

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

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binding/tfjs_backend.cc, line 247 at r1 (raw file):

Previously, dsmilkov (Daniel Smilkov) wrote…

it's a bummer that the TF C API doesn't provide better utilities for constructing string tensors. Looks like their internal storage details are leaking to the user.

Yeah a little funny to use for sure.

[dsmilkov]

LGTM. CI fails due to some lint error

Reviewed 4 of 4 files at r2.
Reviewable status: complete! 1 of 1 approvals obtained