Allow tf.loadModel(url) in node.js if fetch exists

[aman-tiwari]

Description

Check for fetch instead of checking for ENV.get('IS_BROWSER'). This lets people use tf.loadModel('http://....') if they import a fetch polyfill, e.g:

import fetch from 'node-fetch'
global.fetch = fetch

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This change is 

[caisq]

Thanks for the PR @aman-tiwari . This step is necessary, but not sufficient to get http:// loading working in tfjs-node. I will take a detailed look at that feature soon.

[aman-tiwari]

Hey @caisq, not sure what you mean. If one imports the fetch polyfill, this seems to work. One could include the fetch polyfill in tensorflow.js to get it to work without requiring the client to import it, but this seems like the clients responsibility to appropriately prepare the environment.

[aman-tiwari]

Added unit tests as per our in-person conversation.

[nsthorat]

Reviewed 1 of 1 files at r1, 1 of 1 files at r3.
Reviewable status: 0 of 1 approvals obtained