Adds error for building models with negative shapes

[bileschi]

Description

FEATURE: Adds error checking when adding layers to a sequential model which would result in outputs of negative size (matching behavior in py keras.
FIXES: tensorflow/tfjs#318

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This change is 

[caisq]

Thanks!

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

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src/models.ts, line 400 at r1 (raw file):

layer.inboundNodes[0].outputTensors[0].shape.some(

Since the same code is duplicated below, can you refactor the code into a helper method, maybe called "checkShape"?

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src/models.ts, line 401 at r1 (raw file):

x !== null 

I think x !== null can be omitted here, because both null < 0 evaluates to false.

Same below.

[bileschi]

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

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src/models.ts, line 400 at r1 (raw file):

Previously, caisq (Shanqing Cai) wrote…

layer.inboundNodes[0].outputTensors[0].shape.some(

Since the same code is duplicated below, can you refactor the code into a helper method, maybe called "checkShape"?

Done.

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src/models.ts, line 401 at r1 (raw file):

Previously, caisq (Shanqing Cai) wrote…

x !== null 

I think x !== null can be omitted here, because both null < 0 evaluates to false.

Same below.

Done.

[bileschi]

Thanks for the review!

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