gather(): Address indices validation and other algorithm nits

* webmachinelearning#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 webmachinelearning#486

Author: inexorabletash

index.bs

@@ -2714,40 +2714,41 @@ 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 are clamped to 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. Implementations can introduce clamping operands in the compiled graph if the required clamping behavior is not provided by the underlying platform.
+</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.