Update tensor_prod.md

[Spinachboul]

I have made a new PR for the previous change. This is just changing the tensor_prod function in a more readable form which I think I, @ChrisRackauckas and @sathvikbhagavan agreed. Kindly please review it for merging.

Next I am working on explaining the mathematical concepts for Kriging and other surrogate models. I already gave a sample explaination for Kriging and asked if we could go with the same format. So in future I will hit a draft PR where I will constantly modify the tutorials section.

Well this is the explaination for Kriging:

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (b66096e) 78.00% compared to head (af820a3) 78.12%. Report is 11 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #464 +/- ## ========================================== + Coverage 78.00% 78.12% +0.12% ========================================== Files 23 23 Lines 3155 3159 +4 ========================================== + Hits 2461 2468 +7 + Misses 694 691 -3 ```

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[Spinachboul]

@sathvikbhagavan and @ChrisRackauckas Have you gone through the explanation and also the PR??

[sathvikbhagavan]

For the benchmarks, I was thinking each benchmark problem should have three sections:

1. Introduction - explains the problem and provides a mathematical formulation
2. Collect training and test data and some simple plot
3. Trains various surrogates, reports test accuracy, plots for each - and reports which one is the best among them

Can you do the same for tensor product benchmark?

As for the Kriging, please open a separate PR and edit the explanations in the tutorials. It will be easier to see the changes.

[Spinachboul]

@sathvikbhagavan. Could you please look into the PR I have made the changes in tensor_prod benchmark

[Spinachboul]

@ChrisRackauckas and @sathvikbhagavan Please have a look, at this PR. I have used 5 models for benchmark tensor product function

[sathvikbhagavan]

@ChrisRackauckas just to clarify, should we have benchmarks section in Surrogates documentation or completely move them to SciMLBenchmarks?

[ChrisRackauckas]

Yes, we should make a new surrogates section