Add test and guard for circular model weight assignment.

[bileschi]

Description

Fixes tfjs#465 "Manually setting weights for mode" in the case of setting weights to weights the model already owns. Previously the TFJS infrastructure was discarding needed weights.

Adds unit tests covering the behavior (fit model, assign weights to weights the model already owns, check the weights).

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For repository owners only:

BUG: fixes tfjs-union#465

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This change is 

[caisq]

Reviewable status: 0 of 1 approvals obtained (waiting on @bileschi, @caisq, and @ericdnielsen)

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src/variables.ts, line 98 at r1 (raw file):

    if (this.val.id !== newVal.id) {
      this.val.assign(newVal);

You can move the following if statement into this if statement, because if it is the same tensor, there is no need to apply the constraint again.

[bileschi]

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src/variables.ts, line 98 at r1 (raw file):

Previously, caisq (Shanqing Cai) wrote…

    if (this.val.id !== newVal.id) {
      this.val.assign(newVal);

You can move the following if statement into this if statement, because if it is the same tensor, there is no need to apply the constraint again.

Thanks for the review :)

Done.