Add specification for computing a tensor contraction (linalg: tensordot) (#136)

* Add tensordot specification

* Update data type requirements

* Update dtype requirements

* Fix missing header

Author: kgryte

spec/API_specification/linear_algebra_functions.md

@@ -332,6 +332,39 @@ TODO
 
 TODO
 
+(function-tensordot)=
+### tensordot(x1, x2, /, *, axes=2)
+
+Returns a tensor contraction of `x1` and `x2` over specific axes.
+
+#### Parameters
+
+-   **x1**: _<array>_
+
+    -   first input array. Should have a numeric data type.
+
+-   **x2**: _<array>_
+
+    -   second input array. Must be compatible with `x1` (see {ref}`broadcasting`). Should have a numeric data type.
+
+-   **axes**: _Union\[ int, Tuple\[ Sequence\[ int ], Sequence\[ int ] ] ]_
+
+    -   number of axes (dimensions) to contract or explicit sequences of axes (dimensions) for `x1` and `x2`, respectively.
+       
+        If `axes` is an `int` equal to `N`, then contraction must be performed over the last `N` axes of `x1` and the first `N` axes of `x2` in order. The size of each corresponding axis (dimension) must match. Must be nonnegative.
+
+        -   If `N` equals `0`, the result is the tensor (outer) product.
+        -   If `N` equals `1`, the result is the tensor dot product.
+        -   If `N` equals `2`, the result is the tensor double contraction (default).
+       
+        If `axes` is a tuple of two sequences `(x1_axes, x2_axes)`, the first sequence must apply to `x` and the second sequence to `x2`. Both sequences must have the same length. Each axis (dimension) `x1_axes[i]` for `x1` must have the same size as the respective axis (dimension) `x2_axes[i]` for `x2`. Each sequence must consist of unique (nonnegative) integers that specify valid axes for each respective array.
+
+#### Returns
+
+-   **out**: _<array>_
+
+    -   an array containing the tensor contraction whose shape consists of the non-contracted axes (dimensions) of the first array `x1`, followed by the non-contracted axes (dimensions) of the second array `x2`. The returned array must have a data type determined by {ref}`type-promotion`.
+
 (function-trace)=
 ### trace(x, /, *, axis1=0, axis2=1, offset=0)