Adding ability for tensor_expression to use std::ostream

[amitsingh19975]

Currently, we need to construct a new tensor from the tensor expression to output the expression, which is conducive to performance, or the user may not want to construct the tensor. It is also tedious to construct the tensor just to test the expression if the output is correct or not. With minimal changes, we can add the support for tensor_expression to output the contents stored inside the expression.

    using tensor_t = tensor_dynamic<float>;
    auto const e = extents<>{3,3};

    auto const a = tensor_t{e,2.f};
    auto expr = a + tensor_t{e,4.f}; // This will possible after the merge of pull request #160
    std::cout<<expr<<'\n'; // Not possible with current implementation
   
    std::cout<<tensor_t(expr)<<'\n'; // Outputs the expression after constructing a new tensor.