gather(): Address indices validation and other algorithm nits (#642)

* gather(): Address indices validation and other algorithm nits

* #486 points out that indices can't be validated at build-time,
    and clamping behavior with an implementation note is given
    instead.

* Fix a typo in the steps.

* Replace several map-like iterations over lists with list iteration.

Fixes #486

* Update index.bs

Co-authored-by: Ningxin Hu <ningxin.hu@intel.com>

* Add note about negative indices. fixes #484

* Update index.bs

Co-authored-by: Dwayne Robinson <dwayner@microsoft.com>

* Update index.bs

Co-authored-by: Dwayne Robinson <dwayner@microsoft.com>

* Fix grammar glitch

---------

Co-authored-by: Ningxin Hu <ningxin.hu@intel.com>
Co-authored-by: Dwayne Robinson <dwayner@microsoft.com>

index.bs

@@ -2715,40 +2715,41 @@ partial interface MLGraphBuilder {
 <div>
     **Arguments:**
         - *input*: an {{MLOperand}}. The input N-D tensor from which the values are gathered.
-        - *indices*: an {{MLOperand}}. The indices N-D tensor of the input values to gather. The values must be of type {{MLOperandDataType/"uint32"}} or {{MLOperandDataType/"int64"}} in the range [0, N-1] where N is the size of the input dimension indexed by *options.axis*.
+        - *indices*: an {{MLOperand}}. The indices N-D tensor of the input values to gather. The values must be of type {{MLOperandDataType/"uint32"}} or {{MLOperandDataType/"int64"}}, and must be in the range -N (inclusive) to N (exclusive) where N is the size of the input dimension indexed by *options.axis*, and a negative index means indexing from the end of the dimension.
         - *options*: an optional {{MLGatherOptions}}. The optional parameters of the operation.
 
     **Returns:** an {{MLOperand}}. The output N-D tensor of [=MLOperand/rank=] equal to the [=MLOperand/rank=] of *input* + the [=MLOperand/rank=] of *indices* - 1.
 </div>
 
+<div class="note">
+  The {{MLGraphBuilder/gather(input, indices, options)/indices}} parameter to {{MLGraphBuilder/gather()}} can not be clamped to the allowed range when the graph is built because the inputs are not known until execution. Implementations can introduce {{MLGraphBuilder/clamp()}} in the compiled graph if the required clamping behavior is not provided by the underlying platform. Similarly, if the underlying platform does not support negative indices, the implementation can introduce operations in the compiled graph to transform a negative index from the end of the dimension into a positive index.
+</div>
+
 <details open algorithm>
   <summary>
     The <dfn method for=MLGraphBuilder>gather(|input|, |indices|, |options|)</dfn> method steps are:
   </summary>
-    1. If [=MLGraphBuilder/validating operand=] with [=this=] and any of |input| abd |indices| returns false, then [=exception/throw=] a {{TypeError}}.
+    1. If [=MLGraphBuilder/validating operand=] with [=this=] and any of |input| and |indices| returns false, then [=exception/throw=] a {{TypeError}}.
     1. If |indices|'s [=MLOperand/dataType=] is neither {{MLOperandDataType/"uint32"}} nor {{MLOperandDataType/"int64"}}, then [=exception/throw=] a {{TypeError}}.
     1. Let |shapeInput| be |input|'s [=MLOperand/shape=] and |rankInput| be |shapeInput|'s [=MLOperand/rank=].
     1. Let |shapeIndices| be |indices|'s [=MLOperand/shape=].
     1. Let |axis| be |options|.{{MLGatherOptions/axis}}.
-    1. Let |axisSize| be |input|'s [=MLOperand/shape=][|axis|]
     1. If |axis| is greater than or equal to |rankInput|, then [=exception/throw=] a {{TypeError}}.
-    1. [=map/For each=] |index| → |value| of |indices|:
-        1. If |index| is greater than or equal to |axisSize|, then [=exception/throw=] a {{TypeError}}.
     1. Let |dimCount| be zero.
     1. Let |rankOutput| be zero.
     1. Let |shapeOutput| be an empty list.
-    1. [=map/For each=] |size| → |value| of |shapeInput|:
+    1. [=list/For each=] |size| of |shapeInput|:
         1. If |dimCount| is equal to |axis| then [=iteration/break=].
         1. Set |shapeOutput|[|dimCount|] to |size|.
         1. Increment |dimCount| by one.
     1. Set |rankOutput| to |dimCount|.
     1. Let |dimCount| be zero.
-    1. [=map/For each=] |size| → |value| of |shapeIndices|:
+    1. [=list/For each=] |size| of |shapeIndices|:
         1. Set |shapeOutput|[|rankOutput| + |dimCount|] to |size|.
         1. Increment |dimCount| by one.
     1. Set |rankOutput| to |rankOutput| + |dimCount|.
     1. Let |dimCount| be zero.
-    1. [=map/For each=] |size| → |value| of |shapeInput|:
+    1. [=list/For each=] |size| of |shapeInput|:
         1. If |dimCount| is less than or equal to |axis| then [=iteration/continue=].
         1. Set |shapeOutput|[|rankOutput| + |dimCount| - |axis| - 1] to |size|.
         1. Increment |dimCount| by one.