Fixed exception when sigma is already a Tensor object and not numpy array.

[nileshkokane01]

What does this PR do?

Fixes exception when sigma is already a Tensor object and an attempt is made to cast it to tensor again.

Fixes #6054

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@yiyixuxu

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[yiyixuxu]

Hi @nileshkokane01
this PR come in first so we will try to solve the bug with this PR for now:)
#6056 (review)

[nileshkokane01]

Sure!