Fix sample_weight performance problem

[richmanbtc]

Problem

• The performance is degraded due to the wrong way of passing sample_weight to keras api.
• fix model.fit() slow with sample_weight keras-team/keras#16177 (comment)

Solution

• Fixed to pass sample_weight to keras api in the correct way.

[adriangb]

So this "fix" makes legitimate tests fail. I'd also like to see a notebook showing performance degradation and how this fix addresses it.