Improve the comments at the beginning of index_compute.h (#1946)

I just started to learn indexing, and the comment at the beginning of index_compute.h does not look good...

torch/csrc/jit/codegen/cuda/index_compute.h

@@ -17,40 +17,40 @@
  * indices (based on input indices) that match the root dimension.
  *
  * For example with GLOBAL tensor:
- * TV[I, J]
- * TV[Io, Ii{4}, J] = TV.split(I, factor=4)
+ * TV[I, K]
+ * TV[Io, Ii{4}, K] = TV.split(I, factor=4)
  * ALLOC: NONE
  * INDEX: indexCompute {i, j, k} -> {i * 4 + j, k}
- * FLATTENED_INDEX: {i * 4 + j, k} -> {i * 4 + j * J + k}
+ * FLATTENED_INDEX: {i * 4 + j, k} -> {(i * 4 + j) * K + k}
  * PREDICATE: {i * 4 + j, k} -> i * 4 + j < I
  *
  *
  * For example with SHARED tensor:
  *
- * global_TV[I, J]
- * global_TV[Io, Ii{4}, J] = global_TV.split(I, factor=4)
+ * global_TV[I, K]
+ * global_TV[Io, Ii{4}, K] = global_TV.split(I, factor=4)
  * smem_TV.compute_at(global_TV, 1)
  * global_TV.parallelize(1, threadIDx.x)
  *
- * ALLOC: alloc(smem_TV, 4 x J)
+ * ALLOC: alloc(smem_TV, 4 x K)
  * INDEX: indexCompute(smem_TV, {threadIdx.x, k}) -> {threadIdx.x, k}
- * FLATTENED_INDEX: {threadIdx.x * 4 + j, k} -> {threadIdx.x * 4 + j * J + k}
+ * FLATTENED_INDEX: {threadIdx.x * 4 + j, k} -> {(threadIdx.x * 4 + j) * K + k}
  * PREDICATE: {threadIdx.x * 4 + j, k} -> threadIdx.x * 4 + j < I // Same as if
  * global
  *
  *
  * For example with LOCAL tensor:
- * global_TV[I, J, K]
- * global_TV[Io, Ii{4}, J] = global_TV.split(I, factor=4)
- * reg_TV.compute_at(global_TV, 1)
+ * global_TV[I, K, L]
+ * global_TV[Io, Ii{4}, K, L] = global_TV.split(I, factor=4)
+ * reg_TV.compute_at(global_TV, 2)
  * global_TV.parallelize(1, threadIDx.x)
  * global_TV{i, j, k, l} -> { i * 4 + j, k, l }
- * global_TV{ i * 4 + j, k, l } -> { i * 4 + j * J * K  +  k * K  +  l}
+ * global_TV{ i * 4 + j, k, l } -> { (i * 4 + j) * K * L  +  k * L  +  l}
  *
- * ALLOC: alloc(reg_TV, J x K)
+ * ALLOC: alloc(reg_TV, K x L)
  * INDEX: {k, l} -> {k, l}
- * FLATTENED_INDEX: {k, l} -> {k * J + l}
- * PREDICATE: i * 4 + j < I && k < J && l < K ->  // Same as if global
+ * FLATTENED_INDEX: {k, l} -> {k * L + l}
+ * PREDICATE: i * 4 + j < I && k < K && l < L ->  // Same as if global
  *
  * These indices can then be flattened later based on strides.
  */