Dispose variables at once

[Lewuathe]

Purpose

Using same name variable in eager mode can lead memory leak. disposeVariables would be useful to make it possible to reuse same name variables.

see: tensorflow/tfjs#107

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This change is 

[dsmilkov]

Thanks! This is useful!

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Review status: 0 of 2 files reviewed at latest revision, all discussions resolved.

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src/engine_test.ts, line 540 at r1 (raw file):

      tf.tensor1d([1, 2, 3]).variable(true, 'v1');
    }).toThrowError();
    tf.environment.ENV.engine.disposeVariables();

Let's expose this method at the top level so users can call tf.disposeVariables(). Add disposeVariables as a static method to Environment with@doc decorator and expose it in index.ts. See Environment.getBackend() as an example how to expose to the top-level.

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Comments from Reviewable

[nsthorat]

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Review status: 0 of 4 files reviewed at latest revision, 1 unresolved discussion, some commit checks failed.

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Comments from Reviewable