TensMul: ensure that coeff*nocoeff == self

[Kishore96in]

Brief description of what is fixed or changed

Earlier, if doit had not been called on a TensMul, self.coeff would be set to 1, while self.nocoeff would unconditionally drop the non-tensor arguments. This PR ensures that even if doit has not been called, we have self.coeff * self.nocoeff == self

Other comments

I have added a test.

Release Notes

• tensor
  • For any TensMul instance A, ensure that A.coeff*A.nocoeff == A is always true.

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• tensor
  • For any TensMul instance A, ensure that A.coeff*A.nocoeff == A is always true. (#26476 by @Kishore96in)

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#### Brief description of what is fixed or changed

Earlier, if `doit` had not been called on a TensMul, self.coeff would be set to 1, while self.nocoeff would unconditionally drop the non-tensor arguments. This PR ensures that even if doit has not been called, we have `self.coeff * self.nocoeff == self`

#### Other comments

I have added a test.

#### Release Notes

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* tensor
  * For any TensMul instance `A`, ensure that `A.coeff*A.nocoeff == A` is always true.
<!-- END RELEASE NOTES -->