Update saved_model.ts

[rthadur]

Fixes #2698

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This change is 

[Nithanaroy]

Wow! So fast. Thank you.
Do we need a similar mapping for other types supported by Python Tensorflow (e.g float64)? https://www.tensorflow.org/api_docs/python/tf/dtypes/DType

[kangyizhang]

@rthadur thanks for the PR. But this will not work because int64 is not supported in TensorFlow.js (same as other unmentioned datatypes). What we need to do is to mention this in documentation. Can you add a PR to list supported datatypes in the jsdoc (https://github.com/tensorflow/tfjs/blob/master/tfjs-node/src/saved_model.ts#L357), and also update the error message saying that users can modify their model in python to convert the datatype.

[Nithanaroy]

Hi Kangiyi, just for my knowledge why doesn't tfjs support 64 bit computation?
Is it because of no native support in JS (saw from https://developer.mozilla.org/en-US/docs/Mozilla/js-ctypes/js-ctypes_reference/Int64)? Are there plans to in near future?

[rthadur]

@kangyizhang will submit a new PR soon. Thank you

[rthadur]

@kangyizhang I have update this PR , can you please review